Good luck finding 3L Grolsch style bottles...
I think you'd be better off just getting the 1L or 2L sized bottles. I've used the 1L and they work really well. I've been bottling mostly in the 500ml sized bottles, which is a perfect amount for a pint glass. 1L bottles are good for sharing with a couple of friends (one bottle to pop). I wouldn't want to carry a 5L bottle of home brew around. Considering how heavy 5L is, in a 5L mini-keg, a glass bottle would be even heavier (by no small amount either)...
I wouldn't recommend the 5L mini-kegs to anyone at this time... I've used them and they really suck if you're not going to finish them in one, maybe two, days (even on CO2)... They leak gas really badly, and you'll go through more than a couple of cartridges if serving where it's cool/cold (like last weekend where it was in the low 30's for a high)...