How do I even defend myself in 3v1 sword fights?

I'm a returning player (played on launch until I reached end-game at the time where DD was almost only pvp), so my end-game knowledge is very limited.

That being said, I've been having a blast discovering the game again. I'm fully equipped in T6 (green), although only my chestpiece and short blade are unique (weapon grade 1). I'm struggling a lot in fights. I can barely beat the Level 2 bosses in the overlay map stations. Where I really hit a wall starts at the DD D row stations. When those rooms lock in on you, I'm completely hopeless.

While I have a shielded gunner melting my HP, 3 other sword fighters are just attacking from all side and staggering me into oblivion. I can two shots them individually with heavy attacks through their shields, but when it's 3v1, I can't even place a single hit without being stabbed.

I feel like I'm doing something wrong. I looked into my skill tree to see what other abilities could possibly get me out of this jam but I'm not quite sure if there's one.

Any tips would be appreciated!