VPN will terminate on the ASA I assume, so which ISP is connected to the ASA?
If you have a router in front of the ASA that connects to 3 ISP with default gateway pointing towards the 3 ISPs, then I wouldn't think there is any particular way to find out which ISP connection the VPN Client is using. Wouldn't it randomly use any of the 3 ISP? It really depends on how the routing is setup for these 3 ISPs.
There is no way to check how many users VPN in for a month. Unfortunately ASDM does not have that feature.
You can check how many active VPN sessions, and how many total to date (show vpn-sessiondb summary), however, not how many in a month.