Here are three different letter/number sequences. Try to solve as many as possible. The spoilers give hints to each sequence in the form of how many elements are in the corresponding sequence.
Sequence 1 (Missing Elements: 2) [Solved by xnor]
Y, Y, ?, L, Y, E, ?, T, R...
There are 12 elements in this sequence.
Sequence 2 (Missing Elements: 2) [Solved by Tryth]
a, H, R, 0, c, ?, o, ?, L, 3, B...
There are 44 elements in this sequence.
Sequence 3 (Missing Elements: 3) [Solved by finsternis]
?, P, C, P, C, ?, B, R, B, R, ?, R, S, R, S, R...
There are 29 elements in this sequence.
The first one is quite simple but the other two are fairly challenging. Good luck!