This is what happens with the ultralight 3" concepts gets taken to a somewhat ridiculous extreme.

Originally the plan was to use this build to stress test the JHEMCU AIO boards and test out Betaflight 4.3 builds on this class of quads. Intead, I inadvertently created an absolute monster that can run down 5" quads using $8 batteries. The handling and agility on offer from this quad is insane, and for the actual cost of this build, it had me wondering if 5" quadcopters are really worth building for races.

Budget is still a significant consideration, but this is the revised version that gets pretty much everything right, although trimming weight would always be an area to try and improve on.

Motors are the RCINPower 1207 5000KV motors. At the price Pyrodone had these, I just stacked them deep, and these are among the best motors that company has made for 3" applications (the older 1108's flew amazing, but were not very durable). These 1207's perform amazingly well, the realistic downside is that they don't cool terribly well, although some of this has to do with being able to burn down batteries in 80 seconds on this setup.
Frame is a BetaFPV X-Knight 3 (Really a 4" frame). The TPU Pickle canopy is in my favorite purple color.. This combination is lightweight overall, the overall strength of the frame is great, but the tradeoff is the weight. The 4" frame and replaceable arms make this heavier than a true toothpick build, but this is truly a racer at heart, so it needs to shrug off impacts, and this frame has proven it can do so even plastering itself full speed into metal poles with 1408 motors.
AIO Electronics was the initial basis of this build, and the JHEMCU 35A Gen2 AIO boards (F411 with Blackbox) really opens up some options, especially running 3S packs with some current overhead because this is a light build. The AIO is remarkably nice for a budget unit, and running JESC the setup parties (48kHz PWM, 0.25 Startup Power, MedHigh Timing, and Demag Comp on Low). The Betaflight 4.3 tune is straightforward, and I'm running it on DShot300 at 8k/4k Gyro/PID loops, as required for F411 chips. The actual tune is remarkably close to a 5" quad with RPM filtering running, but the lowpass filter can go way up comfortably because 3" stuff resonates at higher pitch. As always, an extra capacitor is worthwhile, in this case a 25V 470uF capacitor gave me all the overhead, especially if I want to run some 4S apcks.
FPV Gear is no longer a budget-driven set of choices... because the performance is just too good to ignore. Foxeer's Predator V5 Nano is an excellent camera, period. The VTX is an PandRC $10 VTX, because it does what I need it to, and actually is the lightest option that actually works with Tramp VTX control. I tried a UnifyProNano but couldn't get SA to work.
Props are Gemfan, which are the best options on T-mount 3". The 3016's are quite excellent for overall grip and performance, but it's at somewhat of an efficiency tradeoff on 3S and 4S batteries. For some tracks, I actually prefer the Gemfan 3018 biblades for the added efficiency, and honestly the sound. This sounds like the angriest flying sewing machine on the planet, and for good measure that noise is audible over the 5" quads that this allows me to pass. The taller 1207 stator can keep those props wound up in turns, and the straight line speed is pretty phenomenal. I feel like the biblades are kinder to the components, but it has somewhat forced me to choose which tune actually works best, as I have developed bespoke tunes for 2S, 3S, and 4S, as this can run all of them well.
Batteries are the interesting part here. I can run 2S 450mAh/650mAh, 3S 450mAh/520mAh/550mAh/650mAH, and I've tried 4S 450/520/550/650mAh packs. On 2S, this feels like a more more planted GTB339, as the extra motor volume seems to cancel out the lower KV on performance, and the overall higher weight just helps ignore the wind. I wouldn't actually recommend 2S for this setup for performance applications, but $4 batteries make for immense fun and I can actually leave this thing pinned for a full two minutes. Even though it's not an ideal racing setup, I do race it like that alot, simply becaue of how much fun it is, and that it forces me to improve lines and precision to maintain momentum.
On 3S packs the speed is monumentally quick, so much so that I can actually jump from my 6S 2250KV 5" race rigs to this and there isn't a big transition to readjust to speed, it's crazy. I do throttle limit slightly, because the 3S packs can hurt themselves supplying that much power, these motors can take all the power the battery makes, the that last 10% of throttle hurts the battery more than really adds thrust, especially with the triblades equipped.
On 4S, I used a lot of throttle limiting (80%), adjusted the PID tune, and finally foud a solid working tune. The real downside is that the batteries get pushed almost as hard as my 1408 rigs, and those flights are over after 90-100 seconds typically, which is in the range that those batteries are taking a beating. The AUW gets close to 200g with 4S 650mAh packs, and tends to make the quad fly more like an 'angry brick', in the sense that it needs more throttle to negotiate turns, but that the pace stays excellent because I'm able to fly it constantly on-throttle. However, the lap time improvements are still small, and less consistent - if I'm pushing for tenths I'm better off running 5" quads, or on 3" only tracks/venues, moving to the GTB339's instead. Overall, the added performance of more expensive 4S batteries just doesn't quite feel worth it, because the 3S 450mAh & 650mAh packs are so much cheaper and flew great out of the box, so those are my preference.

The speed this offer is so impressive, it has to be seen to be believed.

I have a lot of other 3" quads that perform at astounding levels, but a lot of those feel like they're harnessing a particular prop and throwing motor (1408) at the problem until the speed gets there - this does it elegantly, and with the lower pitch props the delivery of power is exceptional. It sounds like the angriest flying sewing machine in history, and flies about how it sounds. It's comically fast, and cheap enough that I'll throw it into race heats with other pilot's 5" quads and accept that I might damage these quads.... by flying into the back of other pilots - because that's how well it performs when I'm doing my part.