Is there any way I can prolong the veg stage to al...

You should do nothing , your idea is the good thing to do but it is too late now. Let her thrive

I think autos can handle some training techniques, but its good to keep it simple if you do. If youre mentioning you're seeing some preflowers...it may do more harm than good to top her at this point. You're at week three so you maybe have between 5-8 weeks left? Roughly. Id say let this one go naturally and maybe fire up another and experiment on that one. Happy growing, I'm excited to watch an in ground auto. ✌️

autos are like not so much on a timer as in a sling shot, think like when they're in veg, they're taking all they can and storing energy (sort of like when you pull a slingshot back) and once they're ready they blow up in flower ( the release of the slingshot ) ... what you can do in veg to prolong the veg is any type of HST to shock the plant so it does stay in veg a bit longer. the other thing you can do is simply not give it enough light (my auto lasted almost 25 full weeks because it was getting less than 4 hours of light by the end of it). End of the day a soil grow needs light and water, but stressing the plant by witholding water will ruin your grow. So any high stress training is your best bet. HST = topping, plucking leaves, fluxing, FIMing .... hope this helps ! 🚀

Unfortunately nothing you can do will delay a Auto going into flowering. Other than giving her less light.. but that will make her grow less and defeat the purpose. Topping and other methods do delay plants however they will not delay the flowering of an autoflower. If you want her to be bigger then you have to think completely opposite and NOT harm her in anyway. Other than minor LST 😎

i topped fimmed and lst automatics, but it doesnt lead to prolong her she went to flower earlier. the only thing i can imagine is defoilation. because the plant want to reproduce the leaves, but iam not sure if she gts bigger ot just prolongign until flowering comes. i would recommend to let her grow and start with another seed. and if you use automatic put her in the final pot after germination. of if shes aphotoperiod you can do the whole stuff like lst supercropping topping fimming and put her outside in a hole