That depends on the serial number of your camera. If it is 170,000 or above, you can use 1/125, micro-sync 0. If it is 169,999 or below, you can use 1/60, micro-sync 15.
If you're in low enough light that you need flashbulbs, it's quite likely that they will put out much more light than ambient light. I ambient light is really bright, you may get a "ghost" image from the non-flash-lit part of the exposure. But the peak of the flash light is very short, so the flash-lit part will freeze action. Same reason electronic flash works OK with the slow X-sync shutter speeds it requires.
Now, we are assuming that the M3 has similar "class M" characteristics to the bulbs that they list. The manual for the Model Y predates the M3. So I'd recommend a few shots on a test roll (taking notes) to see whether the entire frame is equally lit.
Of course, you choose your aperture based on the subject distance, buy dividing the distance into the guide number for the M3B. The guide number will come from the flashbulb box, based on your film speed.
With 400 speed films, you may have more flash power than you can handle, and may not be able to stop down enough for close shots. People used flashbulbs with Kodachrome II (ASA 25), not Tri-X. I'd say you probably want to use nothing faster than ISO 100. (Unless you have an ND filter handy.)
Note that you want blue flashbulbs for color film, you'll get very amber results from clear flashbulbs.
Getting FP class bulbs (GE 6B or or Sylvania FP26B) makes life a lot easier.