first you need to set a route from the A site over the B site to the C site 🙂
Something like this:
bla@routera#
set routing options static route 2.2.2.0/24 next-hop st0.x
on router C you do the oposit
bla@routerc#
set routing options static route 1.1.1.0/24 next-hop st0.x
You need to have policy's in place on both the "hub" and both spokes to accept traffic from eachother