I have three routers, A, B and C. They are connected over VPN:

A <--> IPSec <--> B <--> IPSec <--> C

A - 192.168.1.0/24
B - 192.168.2.0/24
C - 192.168.3.0/24

Can I access from A to C or C to A? With out VPN connection between A <--> IPSec <--> C?

Only address space covered by a policy can go through a tunnel, so you need covering policies or multiple tunnels, expect best performance with A-B, B-C and A-C tunnels though.