There are a number of things to take into acount here.
1) Does router A do any NAT/PAT on packets going through it. If it does it may be easier to terminate VPN's from B, C on A then start new VPN to main router.
2) Processing power of routers. If you use A as a termination point then it needs to VPN not just for users at Site A but also site B & C.
3) Complexity of configuration. I think if you create separate VPN's for each site to the main site your configuration will be easier.
4) Redundancy. At the moment Router A is single point of failure in that if it goes down B & C also lose connectivity. If you were at some future date to have secondary links from B & C it would make sense to have spearate VPN's rather than aggregate via A.
All things being equal i would look to create individual VPN's from each site but this is a recommendation based o what you have supplied. There may be more factors for you to consider.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...