I want to buy two tickets for a concert but they want me to but three and not leave the seat next to me open, why is that?

Never heard of that before. Maybe change where your sitting

It probably has to do with their covid mitigation policy.That doesn't make sense because they already have to have proof of negative covid test.

I've never had this situation before but I would ask the ticket agent for an explanation that may pertain to this event only.

That's so weird.