Hey! Brand new to the whole writing game, so please take everything I say with a huge dollop of salt.
That said, I too have totally struggled with secondary characters, so I understand where you're coming from with this.
I've found that these few things worked really well for me, so maybe try them out and see if you get any ideas?
Imagine that every character in your book is potentially the next main character - If something happens to MC 1, 2 and 3, who would you want to replace them and why? What makes them interesting enough that they could become MC No. 4?
Just like building a good magic system, the limitations of a character are far more interesting than anything else - What do you, as the writer, not like about this character, and how can you turn that into something juicy?
Finally, and possibly most importantly: How does this character's story end? This is particularly important if the side character is filling a function to the story (Mentor, love interest, antagonist etc.) Are your secondary characters written out of the book when they've accomplished their goals beyond what you need them to do in the story? Do we have an idea of where they might end up or what they might be doing? Great example of this is Hot Pie in GOT/ASOIAF. We know that he achieves some aspect of resolution within his own story separate from the main story. He has a function beyond what our MCs need, and it makes it so much easier to root for him in the moment and in retrospect. Ask yourself: In this character's individual journey, where are they and what do they need to be happy/fulfilled.
Hope that helps!