> 3x multiple of revenues

Yes but what is your growth rate of you / your sector. If revenues are set to double next year, the multiple will be higher.

> the offer is low by about 20%

Don't accept the offer. "We don't go back and forth on it" is equivalent to "We think you are naive and will accept the first offer on the table." Also you are right not to value stock as highly as cash. If the offer is stock, heavily discount it, or refuse the stock and accept cash only.