My general portfolio advice is a combination of examples of iteration/development/design work (often the kind of thing you might do in A level), some drawings from life (e.g. still life, figure drawing, technical drawing) and imaginative work (work that demonstrates combining ideas imaginatively). Some places might request a variety of mediums, and this is often course-dependent - in this case you would want evidence of working in a 3D space, ideally digitally but any spacial work i.e. model making, textiles or prop-making, papercraft, pottery etc all show your ability to work in 3D space. Less is more, they want to see your ability to curate your work. Try and cover all of these areas thoroughly within the limitations they have set in their portfolio guidance.
If you have tried to do 3D modelling before - let me know what you've done & I might be able to give some more specific advice!
If you haven't - try downloading Blender and following some basic tutorials to make some really basic models and then start trying those tools on your own to make something simple. You'll see people often recommend the Blender doughnut - I did do this tutorial back when I was learning 3D but personally I didn't find it to be as helpful as other resources out there as it focuses on giving you an overview of all the capabilities of Blender (including simulation, animation etc.) rather than focusing on modelling - I think you might be better off trying specific modelling tutorials, especially if you want to have something to show on your portfolio - common tutorials lecturers will have seen dozens of times so it's good if you can try and do something original even if it's not quite up to the same quality. (By all means show your tutorial work as well, crediting the resources you used on your portfolio) Try CGEssentials or similar channels on youtube.
Worth noting though - I didn't have any digital 3D on my university entry portfolio and it's by no means a hard criteria. Having evidence of working in 3D space is the most important thing - as long as you have some examples of that, even if it's in physical media, demonstrating your core fundamental skills in traditional media is just as key, if not more - depending on the people looking at your portfolio!