I'm a Second year math undergrad with another year left. I want to continue my academics further. Really interested in theoretical physics and Applied mathematics. Can I get into a theoretical physics Phd with math undergrad degree if I ace Physics GRE?
Yes (you might not even need the Physics GRE). Example:
The usual minimum entry requirement is a first-class honours degree, awarded after a four-year course in physics, mathematics or engineering, or a three-year degree together with a one-year postgraduate course on advanced mathematics and theoretical physics. Part III (MMath/MASt) of the Mathematical Tripos provides such a course. Note, however, that entry is competitive and a higher level of preparation may be required for research in some subject areas.
Really interested in theoretical physics and Applied mathematics.
Can I get into a theoretical physics Phd
You might not even need to get into a physics phd. At least in some parts of the world mathematical physics are considered to be a mathematics subjects. From what I understand they do work on physics. Depending on what flavor of physics you are interested, you might be better suited with a math phd.
If you are sure that you want a physics phd you surely have a shot in having one. Usually graduate positions are open to people with related bachelor's but they often specify some degree of general proficency in the related area. You need to look into specifics for each position. Some phd positions have 2 years of teaching in which background material can be covered. Others start working on a thesis right away. The latter might prove more difficult to get in.
One anectode. Apperantly one of the master's students of my proffesor got into a very good physics post-doc after his phd in algebraic topology.