A layer-3 switch can easily be set up as router, so your best choice is to activate routing there and set it as gateway.
If there's a (WAN) router elsewhere you need to be sure that the L3 switches default route points there and that the router has a route for the subnet behind the L3 switch.
ACLs or policies are only required when you need to filter traffic across the L3 switch.
Another option is to bridge the VLANs. Note that without renumbering you still need to route between different subnets even if they live inside the very same L2 segment/VLAN. If you don't want to renumber Server 3's interface you should consider adding a secondary IP address from 10.10.20.0/24 to avoid routing.