Im a starting engineer and I was set a task to design a flotation system for an unmanned device that drops to seabed, and has to pop up back to the surface. The flotation system is only engaged when the system has to ascend back to the surface.
Now what I started off is 2 scuba airbottles that contain air at 3000psi. Then I have a pressure regulator that brings the pressure down to 150psi. After that I have a solenoid valve that lets the air into the airbags that inflate and increase the displacement of the system. Also a pressure sensor is installed for monitoring the pressure in the airbags. Now the problem is that the airbags can sustain only 3psi, and I need a mechanical check valve, that will monitor the pressure inside the airbags and prevent them from over-inflating. But because the system will be ascending, the surrounding pressure will vary. So my question is, will a check valve (with a cracking pressure of 3psi) work with airbags connected to the inlet of the check valve and the outlet being the surrounding water.

You need a relief valve set to 3 psig.  It's not a check valve.

As the bag rises, outside pressure will reduce and the air will try to expand, so to maintain slightly positive buoyancy, you will have to dump air from the bag; just enough and no more.  If you don't, you may go ballistic, or, if you dump too much, you'll start sinking again.

A 1000 lb vehicle with a volume of 100 ft3 would need 84 ft3 of air to reach neutral buoyancy, so you would have to hold the air volume constantly at 84 ft3 plus just enough more to give you some upward force, all the way up to the surface, dumping 406 ft3 of air by the time you arrived.  

A potential problem is that, at depth, (say about 180 ft in the above example) the 3 psig may only give you a very small volume and buoyancy differential and resulting buoyant force to actually effect upward motion by buoyancy means alone.

Buoyancy control at shallow depths is more difficult.

a given volume at the surface would only have the indicated percentages of the original volume at the indicated depth, so would only provide that same percentage of buoyant force.

depth percent
0 m    100
2 m    83
4 m    71
6 m    63
8 m    56
11    50
13    45
15    42
17    38
19    36
21    33
23    31
25    29
27    28
30    26
32    25
34    24
36    23
38    22
40    21
42    20
44    19
46    19
49    18
51    17
53    17
55    16
57    16
59    15

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