PTC Creo Surfacing

I'm modeling a y-joint for pipes. I'm using surfacing because after some trial and error I've found it's easier to make this part aesthetically pleasing in this way. I'm having some trouble filling in the gaps that are shown in the attached image. I've tried simply doing boundary curves between the edges, however, this just results in a plane that's twisted and not close to what I need.

Thanks in advance!