If someone takes my seat without asking then I automatically say No, unless the alternate seat is a clear improvement. Makes no difference who it is. If I am asked politely before seating, and there is a legitimate reason (crutches, little kid with parent, older couple) then I'll usually be OK.

The A seats are the prime seats on the RJs. Don't mess with them. As noted above, in this case 3D should have offered to move to 3F if he wanted to be helpful. The OP's seat was not his to give away.