I'm not entirely sure that's a memory problem, if it where you would get a Windows warning something to the effect of "Virtual Memory too low..." or something like that.
What format are the images in?  JPEG? PNG? RAW?

Not sure why you would be working with images of those sizes, but what happens if you open each one independently, and reduce them in file size (Save them with another name so you don't loose your original images)? Can you then open all 3 at the same time?

What happens if you open them one by one?

----------------------------------
Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.

After much experimenting I realise that it's a restriction of PSP.  PSP also has problems when using large TIFFs, such as - I open a 30MB TIFF (hi-res poster graphics) which takes about 200MB in memory.  Immediately click Save As... and PSP says there is not enough memory.  It must do some fundamental memory/image size check and bail out.  
I have switched to using Corel Draw (Photo Paint) and all is well.  It can do everything I ask of it, change colour depths, display multi images for comparison, and save the images as well (shock horror!).
The lesson learnt?  You can't use a £50 housewives photo package to do a serious job.
Amberleaves.

Glad you figured it out.

----------------------------------
Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.