My 3 kids leave in two different city's with their mothers 700km away from my home-base. How I could properly calculate child support for them? Does it say anything about me seeing the kids at my cost? They do not contribute or the cost never comes off child support payment.

Their distance from you has no bearing on child support.

Your ability to visit or not visit has no bearing on child support.

Child support is set by state law in the state where the support order is issued.
It will normally be based on income and number of kids support is being paid on.