I was happy to see that they seemed fine.

I get drunk easily, and I'm jealous of them whobecause they can drink a lot.

I met up with my ex-co-workers last weekend.

You could also say something like:

I met up with my former coworkers last weekend.

I rarely go to drinking, but we drank, ate, and laughed a lot that day.

"To go drinking" is a very common way of phrasing 飲みに行く in English.

I was happy to see they seemed finehappy.

The word fine as a very neutral sense to it (as in you're not really that invested in weather or not they are happy or not). So, changing it to "happy" or "cheerful" gives a slightly more caring impression.

I get drunk easily, so I'm jealous of thempeople who can drink a lot.

I think connecting the two clauses with "so" makes more sense here, since the first clause provides a reason for the second (「から」みたいです)

I wonder if it's the first time in 3 years.

I think it may have been about 3 years since the last time I met them. 🤔

I was happy to see that they seemed fine.

I get drunk easily,. I'm jealous of thosem who can drink a lot.

I met up with my ex-co-workers last weekend.

"Ex-co-workers" is understandable. However, if you don't wish to use so many hyphens, you can also say "former co-workers".

I wonder if it' was the first time in 3 years.

"It's" is a contraction for "it is" or "it has". It is never used to replace "it was". Since you're writing about a past event, "it was" is expected.

I rarely go to drinking, but we drank, ate, and laughed a lot that day.

"Go drinking" sounds more natural than "go to drink". "Go _____ing" is quite a common expression you will see. For example: "I went shopping", "I like to go running", "I've gone hiking before", etc.

I get drunk easily, so I'm jealous of thempeople who can drink a lot.

A word like "so" helps to transition between different parts of a sentence.