Well as mentioned, it all required patience, my first pull from TC20 was a 4* and the second 3*, then 3rd another 4* then 2 more 3* and then I got Lianna on the 6th pull, now already fully maxed and on my defense team. Since then it has been 3 stars galore! Except a few days ago I got Sabina.
Needless to mention that 4s would not excite me as I have gotten all the classic 4 stars in the game plus a dub of each, all from regular and events summons . So any 4 will now end up being a fodder.
However, my purpose of TC20 is for classic 5stars and it doesn’t cost me any gem to keep running it, so patiently I kept running one round the clock, while other TCs are helping out with fodders for leveling.
Patience is key in this game. So I suggest you keep trying.