There is quite a bit of evidence to suggest the Earthlings aren't that far outclassed by the base Saiyans, at least not more than they usually were, after those three years of training:
- Dr. Gero (or was it #19?) mistook Yamcha for Goku. As we know Dr. Gero calculated Goku, and everybody, increases in those years but only when Goku turned Super Saiyan Dr. Gero admitted his power exceeded those of his calculations. What this does tell us? Goku's base power was within Dr. Gero's calculations and thus the increase Goku had in Namek was everything but abnormal. This ties up perfectly with Krillin assumption, that near death power-ups is the reason why Goku have always been one step ahead of the Earthlings.
- Tenshinhan, and to a less extent Krillin, is astonished how the Super Saiyans and Piccolo far outclassed him but in no instance comment how far outclassed he is by the Saiyans, if he is supposed to be 100,000 and Goku 5,000,000. Adding to it Krillin comments how it sucks not being a Super Saiyan and Tenshinhan, three years before, admits how Goku is now indeed superior to him but only after him achieved Super Saiyan. What these do tell us? The power base Saiyans attain isn't out of realm for the Earthlings to attain, contrary to Super Saiyan power.
- "I didn't let Chaotzu come. To be honest, he isn't strong enough." Tenshinhan informs Goku why Chaotzu isn't among them. Going by this sentence at least Tenshinhan has to be at a level of power that he considers strong enough to help and 100,000 isn't helpful in this situation. Looking at the fight against Freeza someone less than 100,000 is helpful against an half million opponent but useless against someone over 1,000,000. Considering everybody knew the Artificial Humans are supposed stronger than Freeza, going into the battle with a level of 100,000 claiming himself useful is ridiculous.
- Goku and Piccolo considered Krillin, Tenshinhan, etc. useful against the Artificial Humans. If the Earthlings are defenseless against the Artificial Humans Goku would advise them to retreat just like he did to Krillin when facing Babidi, Dabura, etc.
- Last but not least Tenshinhan hold freaking Cell with Shin Kikoho, when this same move (according to the Daizenshuu the Shin Kikoho is equal to regular Kikoho, except it continuously fires the kiai-type Kihoho) didn't even fazed Nappa and only wrecked his armour.
As for my personal opinion the fight in movie 9 between Trunks & Tenshinhan is actually a decent measurement of Tenshinhan power in that period of time. Although I would only make Trunks turn Super Saiyan cause of Shin Kikoho not because of Tenshinhan powering-up.