I have just posted this on a similar thread so thought I'd add it here too...
The way we have been taught at Uni for this kind of thing is to use an A4 porfolio box like
this one with a drop spine so the images can be slid across from one side to the other for easy viewing.
You would use A4 photo paper and re-size the images on it without trimming the paper, always use the same distance from the top to keep the flow nice & neat. The A4 acrylic wallets on the same site in the link above would be used with this kind of box.
You can get these in A3 but they can be cumbersome for people to open it out to view. It really depends on what you are showing tbh.
The only downside I can see with this sort of box is that if you have a series in a set order & the box gets handed around to different people, they can become un-ordered and spoil the flow.
There are many other ways of doing this, i think there may be some portfolio books also on that site...
I took my images to my Uni interview on mounted on black card... I did get in based on my portfolio alone but the tutors kinda joke about people who use black card (like I did!) and say "you are not looking to do an A level!" This came from the course leader who wrote the content so I generally listen
No disrespect to A Level students but they are looking for something to show you can make this level of study.
I hope this helps.