The Dual PentiMedia 3 (aka the PMD-5640) is a generic/ODM laptop that was released in around January 1996. It shares a similar design to the previous PentiMedia II, but with improved specifications. It does appear that there was both a 486 and a Pentium version of this laptop, but the 486 version is likely quite rare.
I don't currently have the specifications for this model, but it should be very similar to the PentiMedia 3+, aka the PMD-5700.
The bottom label on this laptop should either have the model number PMD-5640 or PMD3-586.
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