Why are there so many different vehicles called M3

Because back in the 1930s they started with model M1 and work their way up for each category, i.e.
M1, M2, M3, M4 Medium tank
M1, M2, M3, M4, M5 Light tank
M1, M2, M3, M4 Halftrack
Because of the confusion, they consolidated the numbering system. It does still happen today though. For example the M9 9mm pistol, the M9 Armored Combat Earthmover (ACE), M9 Tank Dozer Blade, but it is not as bad. Some of the "M" numbers never make it off of paper, but the number is still used up.

You learn something new every day on this site!

Ok cool thought they hight have shared the same chassie or something.

You Americans know how to make everything complicated don't you. Like the Bradly how there is the M2 and the M3, i know these are used for different things but would make sence having the one code then having something at the end of it different like M2-C and M2-I or something like that. C for CFV and I for IFV. But I guess I don't come from a military mind LOL.