Routing between Multiple Sites

[scottnguyen]

I am very new to routing between sites and may need better understanding and explanation than others…

1.  I just inherited an environment that lacks a robust Infrastructure (AD / Exchange / and other servers); constant problems
2.  I am a Systems person (more on AD/Exchange) - not Networking guru
3.  I have three sites (10-15 users each) which I plan on connecting together via pfSense's OpenVPN (T1 & DSL on 2 other sites) - currently Cisco but solution provider no longer supports
4.  Site A = 192.168.25.0/24  Site B = 192.168.26.0/24  Site C: = 192.168.27.0/24
5.  My DHCP, DNS will be hosted by my AD Infrastructure; NOT by pfSense
6.  Goal is to use pfSense just as connection between sites, but be able to route between different subnets stated in #4 above

Question:

1.  What do I need to do to ensure this routing between sites function properly between sites?
2.  Will OpenVPN be robust for this solution or something better than pfSense or even package?
3.  Obviously not working with routers, I am a bit lost in this, but with pfSense, I hope to easily do this - high level explanation would be kindly desired

Lastly, I use pfSense at home, at two other locations (so I understand the benefits and ease of use), but never as connecting sites together - thus need your guidance.

Thank you…