他の店員さんに対応してもらっているので大丈夫です。って英語でなんて言うの？

All of these are good examples of explaining that you are already being helped
you would use the he/she if the person thats helping you is nearby so they can see who it is
thanks/thank you is always a nice way to end the sentence as its polite

It is always polite and courteous to say thank you when someone offers you help.
When you are being 'seen to' this means that someone is helping you or serving you. You can use this in other situations too, for example - "My booking is already being seen to" or "Your colleague is already seeing to me, thank you."

If you are 'being served' or 'being attended to' then it means that someone is in the process of looking after you in respect of your particular needs in this particular situation.

The first phrase ' Someone is already helping me, thanks ' is more informal and friendly.
Remember that using 'thanks' is always more informal than the longer 'thank you'.

The second phrase ' He (or 'she') is already helping me right now, thank you. ' Is used when a store employee asks us for help but their colleague is already helping us and is in close proximity to us. We refer to him in the third person 'he' or 'she' or by their name if it is visible on their nametag.

The final way to communicate this ' Your colleague is already assisting you, but thank you anyway. ' Is the most informal and impolite and should only be used in circumstances which are very informal.

A formal way of saying that you are already being helped in a shop, is to say ‘your colleague is already helping me’. The word ‘colleague’ means co-worker or fellow worker. A quicker way to describe the situation is to say, ‘I’m already being served, thanks’, which explains that another member of staff is already serving you.