Move a database (from decomisioned server) to an existing server with heavy ongoing loads
A couple of transactions running every 2-3 minutes and a 60GB database is pretty light IMHO and shouldn't add much load to an existing instance unless it's at the edge of it's capacity.  

A couple of quick questions to take into account.  You are just moving the database not creating a new instance on the "heavily used" machines correct?  On the old instance is this the only database?

I would check RAM/CPU usage on each server.  Pick the lowest usage instance of the three and compare that to the usage on the old instance.  Remember that the CPU/RAM usage for an instance is more than just that for the database (even if it's the only user DB on the instance).  Also that SQL is pretty good at managing multiple databases/workloads.  

In the end your best bet is going to be to use a development machine, make a copy of the database there and load test.  If everything goes well there then move to the lowest use old instance.

If you have the option to add CPU/RAM to the newer servers then that will be a nice safety net.  Move the DB and monitor. If it needs more memory add it, more CPU add that.