Goku During the Android/Cell Sagas.

After Goku returned from Planet Yardrat, he was undoubtedly stronger than Frieza and King Cold and likely stronger than Trunks. In Trunk's timeline, that same version of Goku defeated both Frieza and King Cold and when Trunks tests Goku, Goku has no issue defending himself.

Yet, by the time the Androids come along, Goku is not noticeably stronger despite 3 years of training. Piccolo notes that Vegeta is more or less on par with Goku when he fights Androids 19 and 20 and when Trunks tries to help Vegeta against Android 18, he fairs no better than Vegeta.

So all three Super Saiyans are on roughly the same level before they enter the Hyperbolic Time Chamber and none of them have grown significantly beyond the version of Trunks who could casually beat both Frieza and King Cold but lost easily to the Future Versions of Androids 17 and 18. Why is that?

Well, theory one is that the version of Super Saiyan they are all using simply has a limit that none of them can surpass without special training. This would work with the idea that grade 2 and 3 were required to become more powerful and that Full Power Super Saiyan was the only way to truly break those limits.

That absolutely makes sense for Trunks and Vegeta. Trunks had no one to teach him how to correctly use Super Saiyan and it is implied that the gap between Vegeta attaining Super Saiyan and his fight with Android 20 is short enough that he wouldn't have had time to master it.

For Goku though, it does not. Surely Goku would have learnt in those three years that he was not getting stronger despite training and would have explored new ways to grow in power like he did during his time in the Hyperbolic Time Chamber. Goku is obsessed with overcoming his limits and spending three years failing to do so is unlike him. Unless... Goku wasn't focused on himself at all.

After Trunks tells Goku about a future where he and the others die and Gohan is left to fend for himself, Goku may have made a choice about how he spends the next three years. Instead of using that time to become stronger than a Super Saiyan, he uses it to make Gohan and Piccolo stronger so that if he dies, the future has a fighting chance.

This makes sense for two reasons. One is that Goku is only shown trying to go beyond grade 1 Super Saiyan once Gohan becomes a Super Saiyan in the Hyperbolic Time Chamber and two is that once Goku gets to King Kai after Cell kills him, he spends 7 years becoming stronger than Gohan, Vegeta or even Cell. Goku clearly can push himself beyond limits without knowing there is an enemy out there to motivate him.

So my theory is that Goku could have spent three years actually getting strong enough to defeat the Androids on his own but he picked Gohan's future development instead.