Only

ChiakingdomI gain my weight in only three days.

Oh my goodness.
Suppose you are a guy and you weigh 155 pounds.
If you gain your weight in three days, that means after three days you weigh 310 pounds! You are a Sumo wrestler now!

I want to know the location of ONLY.
Where should I put only if I want to make a stres
on "in three days"?

ChiakingdomI want to know the location of ONLY.Where should I put only if I want to make a streson "in three days"?

Like this:
He went to the casino and lost all of his money in only three days.

Thank you, then what other sentences? How meaning is different?

These are correct, Chiakingdom:

I gained weight in only three days. = The time was surprisingly short.
(X) I gained weight only in three days. — wrong placement of 'only'
I only gained weight in three days. = I accomplished nothing else during that time

However, we normally mention the amount:

I gained 2 kg

Thank you so much!