I have read that only 3 things cause a holosystolic murmur. tricuspid regurg, mitral regurg, and ventricular septal defect. are these the only 3?

Essentially: Technically an lv to RA shunt would also generate a holosystolic murmur. In some ways this could be considered a combination of a vsd and tr, although there is actually a distinct entity involving the atrioventricular septum. (also, from a practical standpoint, it can sometimes be difficult to distinguish a long systolic ejection murmur from a holosystolic murmur.).

Holosystolic: The commonest holosystolic murmurs are those you listed, there are other physiologies possible to cause the finding but they are uncommon.