What is on those 3 partitions? Windows and system on 1, and data on the other 2??
How big is each partition? How much occupied space on each partition?
Any unallocated (unpartitioned) space?
You can't willy nilly "merge" partitions, at least not with Windows own tools.
Open disk management and look at the partitions, left to right. C for windows is probably on the left.
Do you have access to another drive, perhaps external, or to DVD disc backup and could you put stuff there?
Of the 3 partitions, assuming windows is on the left, can you move everything in the middle partition to either C or the far right partition, at least temporarily? The idea being to empty the middle partition.
Post a screen capture of disk management if you can.