growing in 18 gallon rubbermaid with 3 airstones 5 plants and dynagro 7-9-5 with ph balance at 5.9 and 400 watt hp bulb will it work?
Yes that will work.

Depending on how big your plants are, it's gonna get real crowded quick. also, with 5 plants sitting in the lid of the container...it's a bitch to take the lid off, keep the roots safe, and change the solution, check pH, etc. i'd suggest buying an identical 18 gal tub to set the lid and plants in while you are changing water.

another option would be to set the tub on a riser of some sort and install a valve to drain the res. then you could just open the valve, drain the old solution, close the valve, add new solution.

I just harvested 4 plants in a 14 Gal DWC. They came out really great. nice big buds. 4.75 oz dry weight. While it was my best grow so far, i have moved away from the single res and moved to 5 gallon buckets. It's way easier to move the plants around and changing solution is a breeze.

i'm not sure about your nutes. i like the GH 3 part. but the rest of your setup sounds similar to what i just finished.

good luck

Skeet.