Watering every three days sounds pretty infrequent. If your soil is holding moisture for that long you need better drainage. I have to water my 5 gal pots almost every day because there's so much drainage. What are temps?
Nope just used standard potting soil I think it may be underwatering Idk yet I watered well see in a few hours
every three days is more then enough with a plant that small, they dont go through water very quick at this stage. I personally water every 4-6 days depending on stage of growth. To me it looks like your deficient in N and K. thats why you have the yellowing down low on the plant as well as the spotting and necrotic spots on tips of leaves. give it a good feeding and it will show increased growth rates for sure.... A good way to see if the plant needs to be watered is to just pick the pot up and feel how much it weighs. When a plant drinks most of the water in the pot it makes the overall weight go down....a lot. so feel it when you water and when check it 4 days later and you will see the difference. As long as your plants dont become droopy from no water in the soil you are good to go.
I really like Maxi grow powder nutes from general hydro. its only $15 and lasts a long time. If its not available near you i would just try and find a general Veg nute, something high in Nitrogen with some P and K as well.