Trying to fill Column using partial match from another Sheet

Since you start off with an acknowledgment that you are, and I quote, "Not super tech nor Excel savvy," I'm hoping you'll be willing to consider a different approach altogether.

In general, it's a mistake from the perspective of design of databases to have redundant tables of data. But that's what you're seeking to create: a master table and two subordinate tables, all containing at least these three data elements in common: a client number (two variants of that), client name, client phone number.

Redundancy of data is a problem, in that maintenance of coherency between the three requires extra effort, the possibilities of discrepancy (i.e., error) are great unless scrupulous attention is paid to  ensuring data integrity among the three (or more?) sets of data.

You don't explain the reason behind sheets 2 and 3, so for the sake of discussion, let me assume these are clients of varying degrees of importance, the degree of importance perhaps based on something like contract size. In any event, whatever that distinction is, you could reflect that very difference (and more) by simply adding a column to the master data set (sheet 1 in your instance), a column headed perhaps "Priority" or "Echelon" and then assigning a code of some kind that captures the essence of why you've separated them out in the first place. The "Priority Code" could be a simple A-B-C-_____

Then, whenever needed, you could use the FILTER function to extract from that master list all desired data of all those in Group A. But updates of data would always be done one place only--in the master list.