Well if you got 3 Miners you are deep into it already!
I think there is some confusion here:
your node decides which transactions to propagate to the network (and if you use Knots you decide to not propagate spam transactions). But spams are still propagated by other core nodes, so they still get picked up and mined in a block by miners.
There is no such thing as a “core block”, a block is a block. Once a block has been mined (whether it contains spam transactions or not) every node (core or knots) validate it
Makes sense?