Personally i would try for 4 days per week but a 3 day split is enough if your just starting out, you said you have got all your weightgainers multi vits and protein powders so im wondering how much real food is in there but we will get to that later??
As you know diet is the key, your 3 days a week doesnt have to be a full body workout, you can get away with a 3 day split and 3 days a week is sufficiant for a natty. I only say this because I have very little experience in full body workouts so i cant comment on what is best.
It sounds like you have just started working out, is this true?
The key thing to mass building in training is including all the big compound movements such as:
deadlifts
squats
bench
pullups
Lat pulldowns
milltery presses
Barbell curls
Bent over rows
as for what bodyparts to do, well if your doing a 3 day split (which is sufficiant if your just starting out) try not to workout more than 2 bodyparts a day.
So lets set your days out like this as an example
Monday - Chest and Back
Wed - legs
Fri - bi's, tri's & shoulders
I picked mon wed and fri as days off inbetween can aid recovery a bit easier
Just for basics, we will stick with 3 sets of 10s, so lets say were doing monday and lay out a routine like this
Chest
Incline Dumbell press 3 x 10
pec decs (more of a shaper than a mass builder) 3 x 10
Chest dips 3 x 10
Back
SL deadlifts 3 x 10
WG pulldowns/pullups 3 x 10
Bent over rows 3 x 10
If your not sure about what bodypart excersizes do, this is a great place to find out
Exercise & Muscle Directory
this gives you every bodypart every excersize and what they do, study it for a while and you will have a bit more understanding of what the excersizes are and what they do, when you have grasped this, you will even start to think of your own routines in the future and it wont be overwhelming, it will just come to you in time.
Hope this helps, let me know if you want to know anything else, the above is not gospel but its a good example of where to start.
last of all NO, do not even think about steroids yet, not even in the slightest.
How long have you been working out??