Thanos was able to impressively hold his ground against Iron Man, Thor, and Captain America in Thor. Still, there's very little known about Thanos outside of his crusade to balance the universe with the Infinity Stones leaving a lot of questions regarding his canon origins in the MCU, as well as, other personal details.
Infinity War arguably did a decent job establishing Thanos' plan and motivations despite the several loopholes in his logic. Due to sheer will and commitment to his beliefs, he was able able to accomplish what he was set out to do and, by Endgame, he was fully content knowing that he'd fulfilled his part. It's also the reason why he didn't put up a fight as the Avengers ganged up and eventually killed him in the garden. But due to the heroes' time heist, an alternate version of the character from 2014 made his way into the prime MCU timeline who was driven more than ever to defeat his detractors. This iteration did not wield the power of any of the Infinity Stones, and yet he did a decent job fighting against the Avengers' big three.
This might be surprising for some considering that the son of A'lars, an Eternal. At the beginning of Infinity War, he was able to easily take care of Hulk — so much so that the green-rage monster got seemingly traumatized and refused to appear for the rest of the movie. While Thanos had the Power Stone at that point, he didn't use it while fighting the rampaging beast.
With that strength level, it's not outside the realm of possibility that Thanos was able to fend for himself in Endgame, after all, this version of the character was also younger. But it's also worth noting that the 2014 version of the Mad Titan was more ruthless in his ways. The original MCU Thanos wasn't keen on killing anyone unnecessarily; his mission wasn't personal and was only fueled by his strong belief that the snap was what the galaxy needed to thrive. That more logical and mature iteration of the character was gone in Endgame, and in his place was a cruel and straight-up evil villain who didn't care about anything but accomplishing his goal. Knowing that the Avengers weren't going to sit back and let him win motivated him to simply get rid of them. He was no longer holding back; in fact, he was giving it his all to ensure that the heroes no longer posed a threat to his grand plan.
On the flip-side, the Avengers had also just been ambushed by 2014 Thanos' bombing on their HQ prior to Avengers: Endgame's final battle. Everyone survived, but the incident may have very well injured them in various ways and even depleted their energy. Additionally, they had been busy figuring out the logistics of the time heist and then personally executing their plans. This gave Thanos an advantage since he carefully planned his attack against them.
