|No need for subnetting per say. You have to clearly understand the physical setups before you can apply a ip plan.|
Frame Relay means multiple links on one interface.
Layer 3 switch at main really doesn't come into play at layer 3 unless it has a FR interface. Usually you just have a FR router with sub interfaces at HQ. You can't go from FR to dsl so you need dsl/cable at the HQ to connect.
Looks like this:
Site1router sub1<>HQ FR router sub1
Site2router sub1<>HQ FR router sub2
DSL will require VPN preferably site to site vpn.
IP plan would look like this;
HQ = 192.168.0.x
Site1 = 192.168.1.x
Site2 = 192.168.2.x
Site3 = 192.168.3.x
Between HQ sub1 and sub2 and site1 and site2 can be any ip you want to use.
site1 wan 10.10.1.1 to hq wan 10.10.1.2
site2 wan 10.10.2.1 to hq wan 10.10.2.2
dsl site3 isp provided wan ip to dsl hq isp provided wan ip [one end of either should be a static ip for easier setup of the vpn]