You choose 3 separate providers so that they don't have overlapping spectrum. I've used 2 modems on the same provider, but with each modem locked to a differen

You choose 3 separate providers so that they don't have overlapping spectrum. I've used 2 modems on the same provider, but with each modem locked to a different LTE band. This way, if the provider has one nearby tower on band 1 and another tower on band 2, you can use both of them. (Note: If you're going to do this, make sure that you understand LTE carrier aggregation and have confirmed that the 2 bands are not being offered as a CA combination.)