If a 3'' by 3'' square is added at each successive stage, what will be the area of the rectangle at Stage 6, in square inches?

[asy]size(250); real textsize = 10pt;
draw(unitsquare);
draw(shift(1.5*right)*unitsquare);
draw(shift(2.5*right)*unitsquare);
draw(shift(4*right)*unitsquare);
draw(shift(5*right)*unitsquare);
draw(shift(6*right)*unitsquare);
label("Stage 1",(.5,0),S,fontsize(textsize));
label("Stage 2",(2.5,0),S,fontsize(textsize));
label("Stage 3",(5.5,0),S,fontsize(textsize));[/asy]
The number of squares in the rectangles is an arithmetic sequence with first term 1 and common difference 1. Thus, at Stage 6, there will be 6 squares. Since each square has an area of $3 \cdot 3 = 9$ square inches, the total area of the rectangle at Stage 6 is $6 \cdot 9 = \boxed{54}$ square inches.