Equivalent of look up if

Hi,

I was wondering if it was possible to do a kind of If, vlookup equivalent on BI?
I have 4 tables, 1 is related to the three others. The main table is all alumni with their URNs. The other 3 are tables relating to specific activities, volunteering, events and communications. In the 3 smaller tables, a URN can appear multiple times but in the main alumni table, they only appear once.
I need to be able to show the total amount of alumni who appear in at least one of the other three tables. If they appear in all 3 is irrelevant, I just need to be able to show participation. I also need to filter this by current FY. The dates on each of the activity table are linked to my dates table. The main alumni table doesn't contain dates. IF 3000 alumni attended an event, and 2000 volunteered and 1000 were involved in comms, I know that's a total of 6000 instances of participation. But if 1000 of those who attended an event also volunteered, then my count would drop to 5000 which is what I need. Is there a way I can add a column for each activity on my main alumni table? Is that the smartest way to do this? I have used Countrows to attempt this, but don't seem to be able to include the date filter. Apologies if this isn't making sense!