The question of a 3some depends on what you expect from it. Do you expect to just have sex with boths girls at the same time or do you also expect for them to also have sex with each other? The first option is obviously a lot easier to find than the second.
The best way to approach a 3 some is to to select only the first girl yourself. When you are talking to her, ask her to choose the 2nd girl from her friends. The 2nd girl may not be exactly your type but you will have a far more relaxed experience as the girls already know and like each other. Also, If you look at the ads on escort.cz, you will often notice that if a girl does do lesbi show she will list who she will do it with.
The cost for a 3some / 3some with lesbi show is usually 2x the hourly cost. So if one girl cost 1500 for the hour, you would pay 3000 for the hour with 2 girls. Anal usually costs 500 - 800 kc. Those are the prices for privats, nightclubs will be more expensive. 90% of the time when you pay for the hour, you can cum as many times as you can within that hour. If you go to the very cheap 500 - 1000kc an hour privats, you may only be allowed to cum once, always ask the girl before paying.