The dangerous part comes from the fact they are brakes and they play a critical role in doing stopping your 3t truck.
I am ex mechanic and have done many caliper rebuilds in the past. I strongly recommend you either take the calipers or the whole vehicle to a brake expert if you are even slightly not confident in what you are doing with them. Lives depend on your brakes.
Maybe, if your confident enough, get the calipers done professionally and install yourself with new fluid throughout, new pads and as said before regrease your wheel bearings yourself. Get the brake guys to check your rotors too.
And for gods sake, do it in pairs! Both front! Essential!
Could be expensive in the end, but worth every penny.