If it's a choice of the three, then I'd suggest the Dell. It has a bigger SSD and the better of the GPU's and more importantly for your use, a better CPU. From a hardware point of view that's the one I'd take.
With regard to lightweight, that's purely subjective. They may all weigh in and around each other. Will a few ounces here or there make that much difference in a back pack? No. Choose the better Laptop. In this case the Dell. Even though it's slightly more expensive, it's totally worth it.
a) It is a 2-in-1 laptop that will provide increased practicality, and
b) Being a Lenovo, this Yoga comes with the Novo button that will allow you to reset/restore the machine even when you can't access Windows. This may sound like a trivial issue to bring up, but this particular feature (which is unique to Lenovo devices) has saved my sorry butt many times.
You made no mention of any gaming ambitions, so the 2GB 940MX unit should be perfectly adequate for college work. The three laptops fall within .5kg of each other in terms of weight. The ASUS offers the smallest SSD but does come with the newer MX150 GPU. There's nothing wrong with the Dell, but I do believe you'll find the Yoga's flip-design very useful in a crowded lecture hall.
I would rather ask in your home-country computer forum about what people think for these laptops, and what is their experience with post-sales service. With small markets like yours, it is the local distributor who makes the difference, not the brand / model.