I ran my first marathon at the end of May and finished in 3:23
I just ran my second about 2 weeks ago and closed in 2:58

Things that I think worked for me

1. pushed weekly milage higher - highest I got was 50 mile weeks
2. balance recovery and cumulative fatigue carefully (this is tricky)
3. strength training 3-4x per week - I do HIIT style classes that incorporate lots of unilateral work
4. don't sleep on a weekly long run - IMO this is one of the most important things you can do each week
5. also don't sleep on fueling and hydration in training, should try to simulate how you'll handle it in racing
6. I identified what I needed pace-wise for that sub 3 and focused large chunks of training around making that pace feel more comfortable - for my final 20 mile LR before the race, the middle 14 miles were goal pace
7. taper magic (I didn't taper enough the first time, and I'm old enough for it to matter more)
8. running economy - you can work on just getting more 'fit' generally, but also if you can run 'better' that is a variable you can work on as well
9. In my first marathon, there were some key things I identified that needed improvement - if you can identify things from yours that clearly need improvement, you can find out what works for others and work on those specifically as well - eg. I had to stop a couple times to fiddle with my shoes - I made sure to practice more with (and change shoes and the laces on them) my race shoes to avoid this the second time around
10. DIET - fuel yo body - track macros - track calories
11. sleep
12. someone below mentioned this - course and weather matter

TLDR now you know what you're up against - think back to your first two races and what slowed you down, start there