I used a ML-3 for quite a while with my 20D and I also used 2 different Vivitar flash units (and their dedicated modules made for use with my A-1's) so I see no problem EXCEPT your camera won't control the light output. You simply need to shoot in Manual and adjust as necessary.
Yes, the ML-3 and 300TL are both film era TTL (through the lens) Auto Flash, and will always fire at FULL power when used on a DSLR.
This is because TTL flashes read the exposure off the film plane inside the camera. Since Digital SLR's have no film plane, there is no way for the flash to meter the power.
The 300TL also has a manual mode with High (full power) and LO (1/4 power) settings.