Can I set us a service PO or Service Agreement that will have a Max amount say $3,000 and pay multiple smaller invoices against it until it exceeds the MAx amount?

Yes. One way is to set up a regular PO with the item type of "S" and the service type of "A". There is no receiving on the "S" type items, so make sure you have a clearly defined process in place for invoice payment approvals.

There is also a Service Agreement PO which is created through PO25. In this case you get more of a virtual PO... We do you those here , but I am not as familiar with the details of that type of PO.

Hope this helps.