Seamless Group Portrait Composite Editing

I need to integrate three separate portrait shots into an existing group photo so it looks as though everyone was captured together on the day. The goal is a completely natural blend—skin tones, shadows, highlights, depth of field, and even the subtle grain need to match the source image... (Budget: $10 - $30 AUD, Jobs: Adobe Photoshop, Color Grading, Image Consultation, Image Processing, Photo Editing, Photoshop, Photoshop Design)