Trying for baby #3

I have two children and want to add a third to our family. I have mixed feelings about adding a third so I’m hoping to get some advise. I planned on waiting a few years but recently I’ve gotten baby fever sooo bad. I’m also on a weight loss journey. Maybe I’m selfish. But, I don’t want to lose a bunch of weight and then get pregnant again only to gain it all back. My oldest is 5 and my youngest two. By the time we had a third they’d be 6 and 3 years old. I also want to get my tubes tied because 3 is my ideal number of children. I know I want at least one more but how do I know if the timing is right? My MIL was telling me the timing is never right and we should just do it and get it over with. But, my older sister says I’d be dumb to have a third. 😬

What did you ladies do?