are you having fun or what?
3mm for the hulls would be a gurantee that things will crack unless glassed on both sides with 6oz cloth, you are making a recreational kayak,,recreational, so don't go overboard looking for magic bullets or light weight, low cost, easy construction, AND durability,,,won't happen in this lifetime. You could make it with 1/4" luaan on the bottom panels and 1/8" on the side and deck but like they said watch for voids. Sure you could make the deck in 3mm Okoume,,,,and watch the deck wobble down 2" if you press into it with your hands if there was no underdeck reinforcements and deck glass. If you make the wood thinner you get a more flexible panel,,,for large unsupported areas you'll be telling us what happened later,,like the clothes ad guy says "I gurantee it.." Saving $10 in order to add glass/epoxy later doesn't make sense, trust me. my Patuxent 17 with 3mm decks was glassed all over the place once I saw how much it flexed. And the Mill Creek deck is a larger span. Part of the reason for the necessity for the fancy BS1088 Okoume is that it is CONSISTANT and can bend well for the decks.

With 4mm okoume on the sides and deck you'll be making a cadillac construction, go for it. If you go with Luann with minimal voids glass both sides, exterior with 6oz and interior with 4oz you'll have a durable rec. kayak that will last longer than you will be using it is my guess. If you use fancy okoume 4mm on the deck with no glass or reinforcements you'll develop water stain cracks where you placed weight and flexed the wood down enough to hear little cracks. It's not the end of the world, it's not catastrophic, it's not like the wood can't handle the flexing, Okoume is great stuff. If you DON'T put your weight there it won't matter, if you don't let the boat sit around with water in it or compartments closed it won't matter, if you don't mind little cracking sounds it won't matter. But using 3mm as a weight or cost savings in a rec. kayak only to glass it up again is kind of penny wise pound foolish.

The first Mill Creek 16 deck was made in unglassed 3mm ("oohh it's so light",,,crunch) and it survived about 4 yrs of demos but had to get repaired every year before the deck was replaced. Not bad for 3mm. I wouldn't do it.
If you want low cost search the luann piles and use RAKA, if you want light weight don't use tape, control the application of goop, if you want low cost AND durability use more labor and glass ALL interior sections with 4oz,,, if you drink an extra beer before building then make sure your fingertips don't bring epoxy to your face during construction or near a power saw.