Three gigabytes of memory were used (i.e. read/written) sometime in the past by some process(es). They have not been accessed for a while so the kernel decided to put the data on disk, to keep the free RAM high.
You need not to starve on RAM for paging to occur.
When the processes owning it will die.
That might be just some optimization done by the kernel.
There might be a memory leak in a process, 10 MB per week is not among the fiercest ones.
There might be a growing file in /tmp or any tmpfs based file system. The storage area of tmpfs is virtual memory (not any process virtual memory but the OS virtual memory, i.e. RAM + SWAP as Bakunin wrote).

Note also the free memory might be actually used by the kernel, which isn't constrained by process virtual memory rules.

Solaris uses free memory as UFS and NFS cache, so this free memory contains actual data, but it is nevertheless reported as free by vmstat and similar commands, because it is immediately available for processes allocations.