smbakeresq
Explorer
It's not that hard to achieve if you build around maximising AC. My Eldritch Knight had an AC of 21/26 with Shield at level 3 (the party pooled funds to buy the full plate).
It's best to assume that there will be one party member with a stupid-high AC, and design encounters accordingly. One reason I don't go in for "hordes of goblins" when designing low level encounters.
Also, remember at level 3 you can go up to CR 7, so you could have something like a hill giant - CR 5, +8 attack. Owlbears are CR 3 and +7 attack. You could throw a couple of those at the party.
The horde rules work ok, I just have half the horde use help action to help the other half.
To tank you have to make yourself a target. As a player when I play a tank I play a barbarian or play a sentinel build where if they don’t attack me I attack them.