Site to Site to Site VPN's and fortinet??

I've been doing some searching on this site and most information I have found is from at least 2 years ago.  So I'm going to ask again (hope you don't  mind).  I have 3 sites for my company.  A main site with 2 branch offices.  Each site has 10 users.  I have an Active Directory domain in the main site and want to expand that to the 2 remote offices.  All three sites have Cable modem internet access.  Currently there is no web/email hosting internally (but the possibility exists in the future).  Basically I want to create a WAN using hardware site to site to site VPN.  I need content filtering and perimeter AV and anit-spyware/malware protection.  Like I mentioned the posts I've read were from a while back.  Basically I'm looking at Fortinet and wanted to get opions from the experts out there.  I also thought of Sonicwall and Watchguard.  I like the fortinet products because it's an all-in-one type appliance.  I also need to be able to support the vpn connections using dynamic DNS.  I will be putting another domain controller into one site.  I would also like to know if it possible to create VPN links between site A-site B; site A-site C and site B to site C for some redundancy.  Any help, insight, opions are greatly appreciated.