Reference another worksheet using a cell

[Lynxbci3]

I have a main worksheet that pulls data from one of several other worksheets. I would like to be able to type into cell the name of the worksheet that i wish to use. Is this possible?

E.g
In cell A1 you type worksheet1.xls
In Cell A3 the Formula ='[WORKSHEET1.xls]Sheet1'!$A$5
If i then change A1 to workseet2.xls i want the formula in A3 to change to
='[WORKSHEET2.xls]Sheet1'!$A$5

Something like = A3!Sheet1!$A$5 but this doesn't work

thanks in advance