Why cant Goku control Super Saiyan 3?

"more bottles"

JK lol but in all seriousness, I'm not sure whether mastering the ki drain of SSJ3 is even possible without putting it on top of a god form or something. His base form can't handle SSJ3 for too long, it puts too much stress on the body. Besides, at this point Goku has so many better and more reliable alternatives instead of SSJ3. Needless to say, SSJ3 is one of my favorite forms, it's just not that relevant anymore.

Too difficult, mastering ssj3 is impossible

The form has a downside. It's kinda like Super Saiyan Grade 3 (unlucky number I guess) where the form offers a great boost in power at the cost of something else (in Grade 3's case, mobility and in SSJ3's case, stamina.) SSJ3 works for finishing fights quickly, and SS Grade 3 works for immobile/large (think Hirudegarn large, not Broly large) opponents. However, in general battles, they're outperformed by other forms.

As stated by Goku

He was only able to learn SSJ3 bcox he was dead

Which mean Pushing ur limit to learn a form lile SSJ3 which has 0 stability is next to impossible thing just like Super Kaioken

No, Super Kaio-Ken is different. Because in the afterlife you have infinite stamina, SSJ3's weakness, which was heavy stamina drain, wasn't apparent, while Super Kaio-Ken just put crazy amounts of strain on Goku at the time. I believe that at this point in DBS, Super Kaio-Ken would be entirely feasible, it's just outclassed by a lot of other transformations, namely SSB or UI.

I don't think that Mastered Super Saiyan 3 would be possible. I believe it has something to with how you need to strain your ki and push it to the max that makes it hard. As we saw from DBS, Super Saiyan 3 is harder and even more of a strain on the body than even Super Saiyan Blue (Maybe even SSB Kaioken) as Goku could only use it for a second before losing it again, while he transitioned to Super Saiyan god and Blue just as easily as he would Super Saiyan 2. In the battle with Buu, after Goku had used Super Saiyan 3, he was unable to rebuild his ki, no matter how hard he tried. Super Saiyan 4 and Super Saiyan god are superior forms as they make Goku stronger and drain less ki.

Actually, SSJB KKX20 is Super Kaioken, just on top of SSG to stabilize the form and make it more powerful.

Super Saiyan god is god ki which is created through ki control and and pressure. The problem with Super Kaioken was that Super Saiyan 1's energy was so uncontrollable. If you can achieve a god form in Super Saiyan, you can then achieve Kaioken on top which increases your ki.