Hi wh666 You can, but each of those eero (ISP 1->eero A and ISP2->eero B) will generate separate networks.  You will not be able to use those two eero together on the same network.  Likewise your eero C will need to be attached to one or the other network, not both.

If you do want to setup things in this way you would setup the first network and once it is up and running go to the eero app->Settings and scroll down to the bottom.  It will either have an "Add a new network" button or you would need to click Switch Networks first and then the "Add a new network" button, and that will start you on the second one.  I would recommend that you use a different network name/SSID for each network.  The Switch Network button lists the networks by their SSID and also this will help you make sure to connect your devices to the network you want them on.

Now, if you are trying to aggregate these two ISP connections to one network for either reliability or speed purposes, eero cannot do that.  You would need a network aggregation device (one that takes two internet/network feeds and combines them into one, or provides fail over if one of them fails) and set it up as ISPs->aggregator device->gateway/head eero.  We do not have a recommendation for such a device.