The multiple conditions are always OR cases. If your field is a single select drop-down then don't use REGEX matching, but instead do simple Equal To. Example as follows.

1: Field conditional to others Is Equal To Form A Then Show: 1 3 5 7 8 9 12
2: Field conditional to others Is Equal To Form B Then Show: 1 3 5 7 8 9 14
3: Field conditional to others Is Equal To Form C Then Show: 1 2 3 4 5 6 7 8 9 10 11 13

Believe the above should do the trick. Of course with the above change the value matched to be exactly to what the value is for your drop-down select.

Yeah, sometimes is best to have the condition directly on the field in question. Multiple conditions are always "OR" scenarios so they can conflict if they multiple conditions condition the same fields.

Form A and B have less fields than From C. So when the user registers is ok, but when a Form A user edit his profile all the other fields show up and if he does not filed them the changes in the profiles can´t be saved.

Conditions should also be affecting profile edit unless you set it not to (see the Display at bottom of condition and list of locations for it to affect).

As I said these users are going to be in different access groups, so is there any way to create and assign this different fields profiles to separated user groups?

You can place the fields in a tab that is based off view accesslevels and creating custom view accesslevels based off your usergorups, but this won't have any affect on registration.