As @RayButterworth alluded to, character voice is key here, and fortunately, it's not limited to this situation; it's an essential skill for writing dialogue as well.
Everyone speaks differently. Word choice is one aspect of this; do they prefer simple vocabulary, or flowery descriptions? Informal or formal modes of address? Perhaps they're usually quite casual, but get verbose and technical when their main subject of interest comes up. You're probably familiar with most or all of that from writing dialogue; every bit of it applies just as much to first-person narration.
An aspect more specific to narration is focus. Different narrators will notice (and record) different things about the same scene. Are they constantly taking notice of others' appearance? Do they pay attention to relationships, and where other characters stand with each other? Perhaps they hardly bother to mention more than the barest details of anyone they don't know well, but record the nuances of a room's decoration and the surrounding neighbourhood's architecture. Some of this stems from a character's personality, other parts from their personal interests, but it can say a lot about them while being surprisingly subtle at times.
There's also tone. How seriously does each narrator treat the events of the book? Does their narration include constant mental asides, or is it very focused on the moment? Perhaps they narrate as though for a personal record, complete with mental to-do notes, or perhaps they're thinking of a wider audience. One might use the narration to snark at their situation or other characters without being heard, while another simply says such things openly and a third doesn't even think them.
A mix of these can make for very distinct narrators without ever being blatant about it; just the character's name in the chapter title, as you're doing, should be quite sufficient. I would strongly recommend against varying typefaces, regardless of how many or how few narrators you have; either the difference will be small enough to be meaningless, or it'll be too much and you'll just give your readers eyestrain.