I don't think a third striker is needed at this point. If we go on to sign one, it should be an older player who's content with being a rotational option, like Joselu was at Madrid last season. Three very good young players for one position won't work out, it's 99% guaranteed IMO. I'm also skeptical about Zirkzee's versatility, as he hasn't really played on the wings in his career yet. And I doubt Bruno or an in-form Hojlund would be dropped by ETH, and rightly so.
Also, Hojlund has just had one season where he sustained few injuries as well, but he's extremely promising, and Zirkzee has only had two games. There's not enough data overall on our current striker situation yet IMO.
Will Zirkzee take well to being benched, if Hojlund comes back into the team, and starts scoring goals regularly? I doubt it.
How would it affect Hojlund if the opposite happened, and he couldn't force his way back into the team because Zirkzee was in great form?
It's also a big difference that United used to play with 2 strikers in the XI 20+ years ago under Ferguson. You could have 3 or 4 quality options in that case, but for just 1 position, you can't have three out and out number nines, all young and very good, all needing a lot of playtime. And like I said, Zirkzee's versatility is going to be heavily limited by Bruno IMO. He's a world class #10, always available, and is our captain. He's not getting dropped. There will almost certainly be a period this season when Zirkzee doesn't really feature for weeks, because Hojlund hits form.