So that’s two questions in one post 
For the NAT pool, look at this thread - you’ll need to add all addresses to your WAN-facing interface (I’d say a bridge in your case if that should be routed through the two other /30 networks) to use them.
For your default route question I tend to say that your src-nat rule is defined too loosely (i.e. no src-addresses specified) and/or to tight on the out-interface side…
Check the forum and manual for marking connections and routing for dual-wan scenarios, it’s a bit more labor than on other products.
-Chris