Fusing 3 layers: 1 or 2 firings?

I just tried to fuse 3 layers together on a piece about 9.5" x 11". The middle layer is pieced together. I ended up with bubbles, which I guess was the air trapped between my pieces, which weren't perfect as there were small spaces between them. Should I have fused the middle and bottom first, then refused with the third layer? Will this change the look of the piece, making the design elements spread? I'm trying for a crisp, clean line look.

* Bubbles are hard to eliminate completely when you place the two sheets together 'cold' then heat.... the edges tend to tack first, trapping any air in the middle.
separate the cold pieces by putting small glass spacers in the corners. Then the upper pieces will sag into contact in the middle first: later, the spacers will melt and become very hard to see in the finished work, if at all.... and the air will be excluded from the middle outwards. This method works well too if you have any volatiles in your inclusions.

* Thank you, Tony! I'll try that.

* Hi Lee,
Here is another way that should lead to very clean lines and few if any bubbles. It does require two firings.
Firing one... Place cut design pieces on the kiln shelf and cover with uppermost solid sheet of glass. Soak the glass at 1250F for 45minutes or so. After the soak heat to full fuse. The weight of the top sheet of glass will push down on the cut pieces and forcing them together. This should make for very clean lines. The soak at 1250 may help squeeze out any trapped air between the top sheet and the underlying cut pieces.
Firing two... place the cleaned blank from firing one on top of the base layer. Fire at 300F to 1250F and soak for 45 minutes. Proceed on to full fuse. You may require a fibre dam around the perimeter of the glass to prevent the piece from expanding (if it is thicker than 1/4").
Just a suggestion.

* Thanks, Bob, I hadn't thought of doing the top two layers first.

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