Do we need to have three instantiations of AXI master interface in top file if we have 3 AXI masters and then send these three interfaces to 3 AXI agent instantiations using the config_db? Or, how does it work in case of a multi-master scenario to make connections of interface with the DUT?
Yes, to fully verify your DUT, you need one interface in your testbench for each DUT interface. Since your DUT has 3 AXI interfaces, you need 3 AXI interface instances in the testbench, and you add all 3 of them to the uvm_config_db to be passed to each AXI agent.