virtualbox.org

Mpack suggested the tweaks for memory and core count, essentially trying for the point where all three OS's could hopefully fit without starving each other.

Much of sizing a computer for multiple guests is adding the individual guest needs and the host:
Memory for each guest + host's typical nothing-else-running usage <= total host memory.
Cores for each guest + one for the host <= total host cores (not hyperthreads) <-there's wiggle room on this one, not all OS's use all cores all the time. If they all do, stability tanks.
Drives depend on the drive type: if a platter drive, no more than two modern OS's on one platter drive (personal experience). If SSD, fill that sucker up!

A four-core I5 no hyperthreads or i7 with hyperthreads, SSD drive, and 8GB memory could handle both your guests easily, and maybe one more.

Hi Scott,

Please close this case if ever you have an open one. I guess the problem is resolved.
I have upgraded my RAM from 4 GB to 8 GB and i can now run three OS including the host with no problem.

Things seems to be running smoothly as of this writing.

Thanks for the help!

Nice! more memory does help. Glad you're up and running!

thanks for your help mate!i can confirm that my virtual box is running smoothly for days now with no known issues!