Assigning R and S to carbon centres? (Cahn-Ingold-Prelog rules)

Just a quick question, I am very confused on how to determine whether the three stereocentres in this molecule (biotin) are R or S.
I came to the conclusion that stereocentre 1 is R (not sure if I am right), but confused on the other two. I am not even certain on how to assign priorities to the groups, but more confused on how I know where each group would go in 3D space.
Thank you!

At the chiral centre, assign priority to the first atom attached to the chiral centre based on atomic number. If there are 2 groups that are the same, look for the first point of difference and then compare.
If the second highest group is clockwise (right) from the highest group, it is R.
If it then anticlockwise (left), it is L.
When drawing, the lower atomic number is away from the plane
See https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Chirality/Absolute_Configuration_R-S_Sequence_Rules

Thank you for your response, although I already understand the basic rules. I think I worked the first chiral centre out wrong as I had read something that had confused me and I thought I'd remembered the rules wrong.
I think it is S for 1 and 2, but I am unsure on 3. The nitrogen obviously takes priority, but I am not sure which of the two carbons will be next - do I add up the atomic numbers or look for an individual atom that is higher than the rest? sulfur is greater than the others, but adding up the atomic numbers, for that carbon there is an S and 2H (=18). For the other carbon there is C + N + H = 27.

I believe that 1 is "R"

Priority at carbon #1 runs:
Which, if you put the hydrogen to the back, gives order of descending priority clockwise = "R"

Thank you, that is what I did. I just don't understand how I am supposed to know the orientation (ie. where the other groups would be when H is pointing away and is at the back). I don't know how to work out for certain which group would be facing towards me.

Look at the shape of the lines.
A normal line lies in the plane of the paper.
A wedge gets bigger when it gets closer.
A dotted wedge goes away from the viewer (behind the paper plane)
Looking again, I believe that I have made a mistake. Carbon #1 is indeed "S". The missing hydrogen is on the viewer's side of the carbon.