There is no established etiquette for threesomes that I can think of. I think it all depends upon familiarity and chemistry. I have had about 6 threesomes... 2 of the first type and 4 of the second type.
I have had threesomes where there were definite boundaries. For some I could not use the same condom as I changed pussies and that is a downer cos when I was fucking one and the other looked hot and i wanted to fuck the other then changing condoms can ruin the moment. So if there is no chemistry between the two girls then fuck one while you kiss the other or the other is in some way involved like kissing your neck or playing with your balls or sucking the boobs of the other girl. When you are done with the first start foreplay with the other and after you re energise yourself fuck the second one. Dont fuck once in a threesome... it is such a waste.
Also for threesomes take some snack... preferably booze cos after the break they are loose enough for another round.
However I have had threesomes where there is a strong chemistry between the girls. Normally they are bis. And you know it immediately when you start DFKing them and they DFK each other and tongue each other... then you are in for a treat cos they will suck you dick at the same time and take turns to suck you without stopping the momentum to clean your dick. I mean they tongue each other so why will they not suck your dick in turns. It is such a pleasure to have one suck your dick while the other suck your balls and as the dick sucker slides down one side of the shaft the ball sucker slides up the other side of the shaft and changes roles where the dick sucker is now the ball sucker and the ball sucker becomes the dick sucker... that happened only once naturally and I had to request it in my last two threesomes. But requesting is not so much fun as it was when it came naturally. Also when there is chemistry then they have no problems of changing pussy with the same condom because they are hot and they dont want the momentum to be slowed down. and why not because if they suck each others pussy and tongue each other ... then they wont object to fucking both without a change of condom. The best is when you are fucking one and the other gets so turned on and begs you to fuck her and then the former starts to beg after sometime for a refuck and that makes you feel so fucking good.
Those are my experiences... but it is hard to find two gals with chemistry... that is why i ask them if they are bi and if they want to bring their friend they make out with.
Also this does not mean that you will have the same experience because it all depends on how you lead up to the act...