since you want to visit 3 weeks you will have to have get 2 month tourist visa., Which is standard routine thing to get. Now since you get that it's your will how long you wanna be here... Max 59 days. Common sense any body changing ticket pays charges to airline. Also you aren't cutting your stay but staying more so you paying as you stay. Simple make the itenary for 3 weeks ,including hotel booking and return ticket needed for SETV. If you extend just pay flight charges
Once someone said me in bkk, it's not safe here back in 2015!! Yes everywhere there are incidents , lately it seems we need to be careful for ourselves.. it's no more on comfort zone!