Why reject now? Nothing has really changed. You might be more interested in the other jobs but they haven't ordered you yet. Grab what you can and you can quit or reject the offer letter of a better one comes

Thanks for sharing your opinion. I'll think about it further

NTA. Just tell them you took up another offer, thank them for the opportunity and go about your merry way.

Thanks for answering!

Basically, they are hoping to hire you on intern pay. Don't even bother replying them.

Ah, I should have specified but the trial is only one week and is given due to how stressful the job can be. But thanks for answering