I have noticed APC lists their props for electric planes as "electric" or "slowflyer".What type is the best for 3d?
I need a prop for a Reflection Flatout with a Park 400 Outrunner 720kv.

Go with the "electric " prop to be safe. Go to the web site you find the Park motors at and look for a recommended prop size, most have a starting point. Is this a gear systsem, or outrunner? A 400 outrunner brushless will pull a 9 to 11 X 3.8 to 4.5 prop. don't experiment without the recommended start size if possible. Check the prop size while running with a watt meeter. If the motor is pullilng a lot of amps, more or near the motor rateing, go down on length until motor draws slightly less amps than its rateing. This is the only sure way to check what the motor is drawing, is it too much for the controller and battery, is the next following question. A well ballanced system, prop, motor, controller and battery will not produce a lot of heat at any part of the system. None should be hot. Airplane manufacturers should give you a recommended motor size, usually their own. You can go to a upgrade based on the motor rateing. I find AXI and Hacker to be very good products and well worth their money in the long run. Do your research prior to buying a plane and motor system, will save you a lot of bucks in trying to get it right. I fly 15-16 minutes with a hacker 20-22L on a rcx planes Illusion with TP 1320. mostly 3D About the same with a AXI 2212 slightly different airframe. The hacker is happier with a longer prop. Both are great little motors. Enjoy.

Stupidity has a price that is always paid!

I prefer the APC slowflyer props on my 3D foamy. They are lighter than the props labeled electric and both are designed for electric motors.

The rule of thumb I heard was to use the E (electric) props for applications where RPM exceeds 5000, and the SF (Slow Flyer) props under 5000 RPM. You'll get more thrust with the SF but it's not strong enough for the higher RPMs.

slow flyer for 3d... E for pattern and speed...

Ditto on exeters post above.

The electric props have too much weight to be useful for 3D foamy use, in my opinion they are a little too sluggish for 3D flying where you need instantaneous power with no hesitation.....use the slowflyer props for 3D work, they are lighter and RPM changes are faster....which you REALLY need when trying to pull out of a botched hover or to keep that rolling circle going.
I'd start at a 10x3.8 APC slowflyer prop, and work from there.

wazmo's suggestion is good. Slowflyer is good for 5krpm or less. In your case, with 750kv with 3cell ( i assume ), your expected rpm depend on the load is 5500-7000rpm. At low rpm slowflyer with wide paddle is more effective in pushing (look at oscillating fans) air but it starts load up quickly and end up wasting energy rather than turning it into thrust or speed at higher rpm. On the other hand, electric prop has less surface area and not too effective at low rpm and gets better at higher rpm with less wasted energy ( part of the current draw that does not convert to useful mechanical energy ).

In your case, since the rpm is borderline, you can use both so choose based on your application. Either use 20amp multimeter or whatt meter to determine correct size prop. You want to go as large prop as possible without going over the mfg's suggested max current at full throttle.

slowflyer will flex more than electric prop at high rpm which is another reason it is not good for high rpm app.