Jesse began with pressure at the start of Season 3, he demanded Walt cut him in for what was sold and for the new contract. Walt said no, but eventually relented and gave Jesse half the money after Jesse threatened him; but told Jesse he wouldn't work with him. Jesse then went to cook on his own, but Walt understood Jesse would fuck up, and eventually lead the DEA back to him--which Jesse did....like immediately, like the first action Jesse undertook did exactly that. So in order to appease Jesse he gave Jesse what he wanted, he cut him back in and fired Gale. The entire season was about this.
Also yeah, you're right. Combo's death was Jesse trying to sell their own meth. I forgot. The stolen Meth he was trying to peddle to recovery groups lol (I remember watching and thinking 'god damn this guy is such a piece of shit, its amazing everyone likes him). But the reason he wanted to poison Combo's killers is because he found out the kid killed him. At the time, he had no idea those guys were working for Gus; he didn't go to Walt and Gus to confront them, he originally went to Walt to get help to kill them, by poisoning their food. Walt then went to Mike, to ask Mike if he'd do the hit at which point Mike informed Walt that the dealers were working for Gus (He also essentially offered to get rid of Jesse because he was "just a junkie" and said when Fring hears of this, knowing Jesse is a junkie, he's just going to want to kill him). So Walt had Gus broker a peace and demanded Jesse not be killed but Jesse's objection was the kid. Gus saw the divide between Jesse and Walt, and decided to hammer it by killing the kid to provoke Jesse into coming after the dealers anyway, hoping it would show Walt that Jesse was just a reckless junkie and Walt couldn't save him.
Except, Walt did save him. In that moment, Walt chose Jesse over Gus and began the war. So when Walt realized the kind of man Gus was, he defended Jesse. And that is the kind of principle test Jesse failed. Because despite knowing Gus ordered that kid dead, Jesse went to work for him, betraying Walt. All because...again, Gus made him feel like a man. It really betrayed to me the fact that Jesse's principles were more about revenge than anything else, and ignored the moment they needed to be.