Is there a rule-of-thumb for bathroom sharing one supply line? Should the toilet be at the begging or end of the supply? I think, if it were at the beginning, it would suck the cold water from the shower... like we see on TV.
Does your shower have an anti-scald mixer on it? If it doesn't it should, that will take care of getting a steam shower when flushing. The rest should not make a difference...worse case would be that you might have a slight pressure/volume drop IF all 3 are running at the same time. You could always feed the bath room group with 1/2 to the shower, then step down to 3/8" to the rest...but don't really think that's necessary.
when it comes to breakfast, the chicken is dedicated, the pig is committed.