Here's what I have so far
1. Goku’s Universal Spirit Bomb
The Spirit Bomb used against Kid Buu is explicitly described as a sphere formed from the energy of the entire universe. This implies, at the very least, the sum of the energy of all existing celestial bodies.
The total estimated energy of all stars in the observable universe is approximately 5.0058 × 10⁵⁷ joules, which corresponds to a multi–solar system level.
Goku not only manages to contain, push, and direct this amount of energy, but also withstand its recoil, which indicates that his own energy capacity is comparable or even greater.
2. Elder Kai’s Statement: Buutenks and the Destruction of the Universe
The Elder Kai states that Super Buu with Gotenks absorbed will destroy the entire universe, including the Realm of the Kais.
He also mentions that he has only 1,000 years of life remaining, implying that Buutenks could accomplish this destruction within that time span.
Modeling the destruction of the observable universe over a period of 1,000 years results in a total energy of approximately 8.95688 × 10⁸¹ joules, which corresponds to a multi-galactic level.
3. Dimensional Creation by Marba (Dragon Ball Daima)
Marba creates a pocket dimension that contains a fully star-filled sky, implying the direct creation of multiple stars and planetary systems.
This feat is classified as multi–solar system level.
4. Direct Universal Threat: Kid Buu
During the fight against Kid Buu, it is repeatedly stated that if the heroes fail, the entire universe will be destroyed, and that this battle determines the fate of the whole cosmos.
This establishes Kid Buu as a universal-level threat.
(Now, this is all speculation based on leaks and the trailer, so don't take this point too seriously.
5. Scaling with Dragon Ball Super (Remake)
In Dragon Ball Super: Beerus, the new DBS remake series, when Goku transforms into SSJ3 in front of Beerus, his power makes the entire Other World tremble.
Calculations of this feat give an approximate value of 7.4110983 × 10⁷⁷ joules, which corresponds to a multi-galactic level.
Now, at the beginning of Dragon Ball Super, Goku considers it possible to fight Kid Buu using only SSJ1, something he could only do using SSJ3 during the Majin Buu saga.
This indicates that Goku’s SSJ1 in Super is comparable to Goku’s SSJ3 in the Buu saga.
Since SSJ3 multiplies SSJ1 by 8, it follows that Goku SSJ3 in the Majin Buu saga equals 1/8 of Goku SSJ3 at the beginning of Super.
Final Value of Goku SSJ3 – Majin Buu Saga
If Goku SSJ3 at the beginning of Dragon Ball Super reaches approximately 7.4110983 × 10⁷⁷ joules, then Goku SSJ3 in the Majin Buu saga would have a value close to 9.26 × 10⁷⁶ joules.
Conclusion:
Goku SSJ3 in the Majin Buu saga reaches a multi-galactic energy level, confirming that the characters of this saga far exceed the solar system level and reach galactic, multi-galactic, and even universal scales.