Water supply and pressure are one system. Waste drainage is separate.

Maybe 3? Not all at the same time.

Are you on a septic system? It would take more than a few tissues to do damage.

Tissues pretty much disintigrate when wet (ever tried to clean something wet with one?)... three tissues are not going be a disaster, even if you have septic. Even all flushed at once, I'm not sure they'd create a clog. If you had a clog, your toilet would be emptying poorly or backing up (and the solution would be a plunger).

Yes, it is normal for water pressure at any single faucet to be reduced if you open all the taps at once; that's just physics.

If you have a septic system, you might have a problem over time, but even septic systems can handle the correct type of tissue. I have a regular sewer system and have been throwing tissues into my toilets for 50 years and have never had a problem yet. I guess if you had a huge number of tissues and wadded them up into a big ball you might have a problem...just like you would have with a huge wad of toilet paper.

And, as pointed out above, this has nothing to do with water pressure. When you have multiple taps running, you're putting a demand on your supply to provide more volume, and the flow won't be as strong as when you only have one tap open. This is true to some extent in almost all domestic plumbing unless the supply pipes are large, the pipes aren't internally corroded, and your water pressure is strong.

... that's all assuming it was a paper tissue and not a wipe.