Can I run migration assistant multiple times for multiple computers?

Solution 1:

Without the source or the full functionnal specification from Apple it is impossible to give you an answer with no risk.

To clarify your question let's say your 3 Macs are named A, B and C. A being the oldest one, then B, and C the younger one.

Your question is "can I run Migration Assistant this way":

A → C
and then
B → C

I am convinced that Apple didn't take into consideration such a scheme of migration which might lead to data loss or corruption.

On the other hand, if you want to run Migration Assistant this way:

A → B
and then
B → C

I am convinced that the integration process will work correctly on B and then on C. In this case, make a full backup of B before starting.