Confusion choosing Shimano Derailleur

[mp1156]

I'm struggling to choose a Shimano Derailleur to upgrade my current setup. I currently have a 6x3 18 speed, 14-28T setup with a Shimano Tourney TY-21 Derailleur and SHIMANO TOURNEY ST-EF41 TY EZ FIRE Plus Shift/Brake lever combined shifters (I upgraded this from the Revoshift).
If it helps I have 3 chainrings on the front- 24-42T

I realised to get a better derailleur from a higher group, they come at 7,8 or 9 speeds. From what I've heard (correct me if I'm wrong), all Shimano 7,8,9 Speed RD's are compatible with each other and all shifters in the MTB group?

Here are my 3 options that I've decided:
Shimano Altus RD-M310
Shimano Acera RD-M360
Shimano Acera RD-M3020

Please let me know what would be the best derailleur out of the 3 and that would work with my setup. 

[Brucey]

they all have the same shift ratio so they should index OK using your shifter. After that, it is simply a question of price vs life expectancy. Some folk fall off so often that they rarely get to wear a RD out, making it fairly pointless getting a spendy rear mech, since they will bend just the same, regardless of cost.

[TheBomber]

While it is true that a RM from higher up the food chain can sometimes solve shifting issues, if your gears aren’t working to your liking it might be worth discussing what the problems are first before leaping in with a new RM. You may be about to waste some money.