Demo how to brew beer...and leave it at that. Don't have them taste what you brewed in three days.
Or is you insist on a taste test...have them sample the wort before you pitch the yeast...and tell them that this is what beer taste like before the yeasties do their magic...and since they know what beer tastes like...they can see how important the yeast and the conditioning process are to the final project.
Don't forget SOMETIMES fermentation doesn't even begin until 72 hours...it's called lag time....
This isn't making koolaid, it's a natural living process....we are not in charge, the yeast are. Three days just isn't enough time...and honestly if you handed them subpar beer, let alone bottle bombs...then you would be doing a disservice to homebrewing, yourself and beer culture as a whole....
If you are serving green, yeasty, and nasty tasting beer to people who have never tasted homebrew then they won't understand..what it's supposed to taste like....
They will think that EITHER you suck as a brewer, ALL HOMEBREW SUCKS (and you'll prolly go blind anyway) or those BMC commercials were right, anything other than fizzy yellow beer, especially homebrew taste like a$$, and we should stick to bud light..."THat's what TV says, so it must be true, right?"
You won't be a great ambassador to the world of homebrewing presenting 3 day old beer....
We get variations of this all the time, someone wanting to rush the process so people at a party or gathering can taste the beer....
I don't understand why, if you are hauling homebrewing equipment and ingredients to your demo, you can't also bring a keg, or case of your beer...or even a nice micro in the same style that you brewed....
I understand and applaud you wanting to demo brewing...but either bring some homebrew, or micro beer to sample...or just leave the tasting part out of the demo....but don't have them taste 3 day wort...it won't be pleasant, and really won't be good for erasing all the negative press we have gotten since Anheiser Bush came on the market.