Please click the text link for "Profile" at the top of this page, below the blue bar and update your signature to include your AC version, etc.
I'm a little confused, because your profile says you're in California, but the US library for AC 15 does not have a "Basic Window W3 15" in it that I can find. Are you perhaps using a 3rd party product such as Door/Window Builder?
There is no standard window in the US lib that has a fixed center with casements to either side. But, you can create such a window but ganging together three windows. See the ganging options in the General panel of parameters as shown in the attached. In the screenshot, I've checked both side checkboxes as would be done for the fixed center window.
Not sure if what I show is what you want (hinge sides) but you can see that the ganging of the two casements and center fixed looks OK. I just used default params for trim/frame size - not what I would really use.
Cheers,
Karl
NB to readers: the ganging feature appears to be fixed in 15: there is no extra line where the two windows join.
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB