I am assuming you are talking about an outdoor enclosure. If you set up an appropriate turtle enclosure with a higher fence and /or netting over the top, the dragons will love it. Just have some good sized branches overhanging the water and some basking rocks over hanging the water’s edge and the dragons will be happy.
Remember your turtles need to get out of the water to bask and dry out periodically. So organise some logs jutting out of the water or a commercial turtle dock. Also ensure you have a vey shallow and gradual entry and exit for the turtle to crawl out without scratching its plastron. There are some water proof carpets or artificial carpets you can use for looks or a length of quality rubber pond liner.
I think you will find the biggest issue is having sufficient volume of water to maintain good water quality for three turtles. Bear in mind that they will continue to grow and will need a volume appropriate to whatever size they ultimately reach. You will need a good filter (biological and physical) but not sterilising like UVC units). Addition of submerged plants, such as Ribbon Weed (Vallisneria), submerged & emergent such as Water Milfoil (Myriaphyllum spp) and emergents such as Bullrushes (Typhus spp) will help considerably to maintain water quality and look great besides. Grow the plants in large black plastic pots, several layers of shade cloth in the bottom to contain the potting mix with a covering of smooth gravel on top. When they start to get a bit large, pull them out, chop them back by half or whatever and replace.
Good luck
Blue
