It's very likely that you can't use the three lines as one aggregated big pipe. For that to work all three line have to come from the same ISP and there has to be a product offered by the ISP for that. In that case you would get one common IP-block for all lines and not one per connection.
You could improve the availability in the following way:
1) Build a FO-system with two ASAs.
2) Configure three outside interfaces for backup-ISP-operation. One interface has to be choosen to use the active default route for all your outgoing traffic.
3) Use the two other links for your incoming traffic. Both links can be used simultaniously.
4) The other links can also be used for the VPN-connection in a active/backup-manner.
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