Start to Finish Relationship - earliest start when connecting to two predecessors

If Task C is on the critical path (Has 0 days float/total slack) you should be able to link both Task A and B to Task C using the default FInish-Start relationship type, and set the Constraint Type to "As Late As Possible" for both Task A and B. This will move them as late as they can occur without pushing back the start of Task C. However, If Task C has float/slack, doing this will push Task C as late as possible as well. Alternatively, you can set a deadline for Task C (by double clicking on the Task) and linking A and B to C as described above. If Task A, B, or C falls behind schedule, the dates will still get pushed beyond the deadline, but you will have negative float to alert you of the problem. I'm fairly new to Project, so I hope I understood your question correctly and hope this helps.