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 are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...