There are simmers who feel the same way as you and try to keep their games more realistic. I personally play a variety of saves; some are as realistic as I can make them, while others are fantasy/fairy tale type saves. As you’ve discovered, mods will help you adjust or tune out the features that you don’t want in your game. I personally like Generations overall—for the objects like the treehouse, video cameras and costume chest, the after-school activities, the day care career, and other interactions like teaching teens to drive, asking a parent-in-law-to-be for a Sim’s hand in marriage, pulling pranks, taking infants and tots for stroller rides, etc. It adds those little details to my families’ lives which I enjoy. There are aspects of Generations I can't stand, such as the autonomous and often nonsensical scolding, but I’ve modded things to suit my preferences. NRaas Retuner is highly recommended for tweaking features to suit your style of play.
I think all 3 EP’s you’re currently considering are among the best TS3 has to offer. I like them all for various reasons. However, Ambitions offers Simbots, Seasons brings back aliens, and Uni has PlantSims; so you may want to learn how to tune out or avoid those as needed to keep to the more realistic side of things. Other than that, deciding which EP you’d like to purchase next will depend heavily on what theme you would most like to add to your gameplay experience.
Ambitions is indispensible to me because of the self-employed feature and the ability to consign goods/crafted items. It adds a new world, new careers and professions, new traits and skills, the real estate system, laundry, etc. Being able to blow things up is also fun, lol. Seasons comes with weather and a holiday to celebrate for each season, festivals and contests like pie-eating and apple bobbing. Plus a ton of objects and activities for each season, including roller-skating, fireworks, pumpkin-carving and snowboarding. University has the new world/campus to travel to and several majors to pursue there. There are new careers, traits and skills, and a lot of new objects and activities from bowling and ping pong to bonfires and juice kegs, which you can add to your home town so they’re not limited to university living.
Reading up on all the EP features or watching reviews and walkthroughs may help you decide which packs would appeal to you most, if you haven’t done so yet. The following links have a basic run-down of features included to start you off:
http://sims.wikia.com/wiki/The_Sims_3:_Ambitions
http://sims.wikia.com/wiki/The_Sims_3:_Seasons
http://sims.wikia.com/wiki/The_Sims_3:_University_Life