I wanted to create handles which make Victorias thisgh/buttocks flat. Default bend looks weird when I she sit.

On the picture you can see difference.

I've added child bones to hip, right and left thigh, but all of them must be moved separately. Please advice how can I move 3 bones changing only one parameter.

If you have P-8 or pro 2010 you can use the dependent parameters and teach them to move with the thigh. Start by bending the thigh to the desired bend, then at the dial for bend click edit dependencies. click the start teaching button.  Then dial in your bones to the desired setting. after all is set zero everything and click stop teaching.

you can set slave ERC code from whatever you want as the master dial.

You leave the master dial alone and add this code to the slave channel below the line interpstyle locked

valueOpDeltaAdd
                Figure 1
                BODY:1
                Ybend
            deltaAddDelta 1

ybend is the name of the master dial , BODY is the bodypart where the dial is located

deltaadddelta 1 is the amount to change the slave for very 1 unit change in the master so any fraction can be used like 0.5, 1.6 etc or negative values

Theres a document on ERC code that will work in any Poser version.

http://www.atlantis23.com/erc_download.html

Thanks for answer. I'll try tomorrow or next week and let you know what happend.

Merry Christmas