Reselling bundled games - how viable it is?

you only make $5 in your example. Not including what the grey market site scalps off of that, which is probably more than 10%.
profitz(tm) = returns - investment.

When buying 3 games for 10$ and selling them for 15$ you made 5$ ... Minus the fees for 15$.

Furthermore depending on where you live, you might need to pay taxes if done on a large scale.

And before I sell my games on g2a I rather give them away...

You now have 15$ in virtual cash on some greymarket site. If you want to buy games on that site for 15$ that works okay. If you want to turn that virtual cash back in real cash you will run in some problems like greymarket sites often only allowing powersellers/affiliates/whatever they call it to pay out their money in the first place and also stuff like a certain minimum amount of money available in virtual cash and a handling fee. Also sites like kinguin make you wait 7 days after the game is sold before you even get your virtual cash.

Between handling fee and the site taking its cuts from the sell price, i doubt you will make a big plus if any and certainly not worth it if you take the time invested in consideration.

Don't forget to factor in the scammers as well. Profit = $0

It's possible to make some cash via eBay this way, although their policies are really antiquated, and they go through and remove ALL sales of online-licensed games (i.e., anything that doesn't include a physical disc). I've paid for a bit of my bundle habit that way, although eventually I had some auctions removed, and after talking to eBay support, found that they just don't know how to interpret what's legit and what's not, so they treat all non-physical video game sales as pirated and remove them. There's no fallout so long as you haven't been warned multiple times.

Apparently, if you can prove that you own the license and have the right to resell it and put this language in your auction, then they'll allow it. But it seems like such a narrow loophole that I decided it wasn't worth the effort and/or risk.

It's better to just give away stuff you don't want to get better status and have access to high-return giveaways yourself, I think. :)

When there was a Humble best of 2016, I bought 3 times BTA,because of rust. Now I can easily sell each rust on g2a. (already sold 2/3 rusts)

You can usually sell the top game (early unlock) game from a Humble Monthly on G2A for 100-200% of the bundle price. Total War: Warhammer sells for more than 20 bucks there. And you can cash out from G2A. I'm not promoting them, just stating the facts. For some reason there are people who buy the big game of HM on G2A for twice the price. instead of buying the whole bundle for the right price. So yeah, there's room for a little profit there, at least with Humble Monthly.