Why can't vegeta or Gohan go SS3

Vegeta skipped super Saiyan 3 in GT and Super and achieved higher power. Gohan I thought was tougher by holy ho to do it. How come Trunks and Goten are Able to go super saiyan 3

What's funny is neither Goten or Trunks can do it. Only Gotenks. I don't think the boys have even achieved SSJ2.

The SSJ3 is useless like the USSJ, a tiny example, by the USSJ, the transformation not only increments power while losing speed, it wastes a lot of energy while the SSJ2 increments the power, it increments speed and doesn't waste energy like the USSJ, now the SSJ3 increments speeds and power beyond the SSJ2 but it clearly resides on one of the USSJ's weakness which is a huge waste of energy, I could even say that the SSJ3 waste more energy than the USSJ.

The SSJ3 is for quick finishers, which is risky and not smart at all, based on a tactical vision, it's useless like the USSJ, that's the reason why Vegeta or Gohan never proceeded in obtaining the SSJ3.

Honorable mentions:

One of Goku's reasons why he never proceeded in perfecting the SSJ3 is because there is none alternative option in this transformation, the SSJ3 is like the USSJ, it's a pre-transformation to a legit one like the SSJ2's and SSJ's legit positions, in theory, the SSJ4 would take the SSJ2's legit position, because there won't be a fault anymore.

1. SSJ = Legit
2. USSJ = Not legit
3. SSJ2 = Legit
4. SSJ3 = Not legit
5. SSJ4 = Legit (In theory)

They'd look silly with longer hair.

That could actually be the reason.

Toriyama didn't know how to draw them.

They can go SSJ3, they just have superior transformations that make it obsolete now. Same reason we didn't see Vegeta or Teen Gohan go USSJ.

my guess: there is an offscreen USSJ2 form, that after you reach it and push pass it, you get SSJ3. Goku was able to do it because he was dead and didn't notice the strain, but when Vegeta reach USSJ2, he decided it's not worth it, not knowing there is SSJ3 after that. Gohan didn't train in 7 years. In Super, Blue is superior in every way so who care. Also Gohan still didn't train and use his mystic form instead.

I doubt Vegeta has ever actually wanted the form it's limitations are to great.

Gohan is just to weak because he's a weakling with a crybaby disorder. "I don't want to fight" I mean seriously grow some balls your a saiyan warrior

Vegeta skipped past this form when he reached godhood. Gohan has mystic. This is a completely different type of transformation just like blue form.

They lack the will of the warrior.

They both have achieved forms that make it entirely useless for them. Super Saiyan 3; while great in power, is a tremendous energy drainer. Gohan's ultimate form is a far superior power up. It gave him the power that initially surpassed Goku's Super Saiyan 3 during the Buu arc, without the drawback of the constant loss of energy. SSB is essentially the same thing for Vegeta; massive power up with better ki control. They don't need it.