How does starting a double/triple major process being after you receive a B.S. in one major already?

If you have actually received the BS degree, you are graduated from that program. If, however, you have fulfilled all of the requirements for the degree but still have time left in school, you can take more courses until it is time to graduate. If you are going to then enter the EE MS program, you will have to apply for that. Adding a second undergraduate major may depend on the specific requirements of the department, although you may be able to add computer classes as electives as part of your EE degree. You can get credit for the prerequisites that overlap between EE and the other programs, but not all classes are given every semester, so you will probably have to spend at least 2 more years getting the other major. In any case, what you are thinking about is unusual, you will need to speak with the admissions department and/or your advisor to see what is possible.

Do not double major. Learn as much physics and/or computer engineering as you can, through your electives. A double major is a waste of time and money. As for your MS degree, consider looking for work as soon as you have the BSEE -- and then trying to get your employer to pay at least a part of the cost of your MS study.

Talk to the admissions councilors. This is why they are there.

Nitty gritty is EVERY double major is viewed by the majority of employers as unfocused. Very very few benefit from being a double major. .