X-3 diving in Dr. Ventures circulatory system.
The X-3 is the personal submarine for the Venture Family. It was likely built by Jonas Venture Sr. following the X-1 and X-2. It was kept at the underground submarine bay at The Venture Compound.
The X-3 was first referenced when Billy Quizboy suggests they shrink a submarine and enter Dr. Venture to diagnose an unknown medical problem. Dean says they have one but when they go to the submarine bay it is missing. It is later discovered already inside of Dr. Venture's vein surrounded by blood clot and causing the medical problem. The past clones of Hank and Dean, referred to by Brock as "The Sevens", had shrunk themselves in the X-3 and piloted it into their dad and died in the submarine. As noted by Brock, they left the parking break on. After clearing the clot they are able to pilot the submarine eventually exiting Dr. Venture.