working with rotation matrices

I have a rotation matrices, and I have to do a few things with it.

If the matrix represents the rotations around three axis,

1) how do you apply the rotation around 1 axis to another matrix ?

for example : I want one matrix to have only the y-rotation of the other matrix. For example, when I want a 3rd person camera.

2) how do you set the rotation around a specific axis to a value. But without touching the rotations around the other axis ?

For example, when I want a camera to look at a specific height.

Thanks in advance.