It’s fine, I’m here to always answer stuff and make posts lol
So from what I understand you want to either slide from the 3rd fret on the e string so the 15th fret E string or you just want to play both notes really quickly
I actually took out my guitar and tried to do it (what song requires this omg) but it’s physically impossible to slide from the low e string to the high E string like unless you’re using a bottleneck or your entire finger so I suggest jumping from string to string. I mean I’ve been soloing for a bit, and what I do when I have long stretches like that (The Unplugged version of the Layla solo by Eric Clapton has one jump from the 5th fret D string to 13th fret E string and I just jump).
Just practice it slow is all I can really suggest maybe just with your middle finger on the low e 3rd fret, don’t let your hand come up too far off the fret board and attack the E string on the 15th fret. In fact don’t be so extreme at first. Take it easy do 3rd fret e string to 3rd fret E, then 3rd fret e to 4th fret E etc…
Idk I think jumping like that just comes in feeling the rhythm and soloing like in the Layla example. Check out the way this guy does it. It isn’t v smooth, but it’s something.
I hope I was of some help, and if you feel like I could be clearer definitely feel free to ask, I love helping; you guys don’t know:D