There are pros and cons for both options.
If you use separate 15m and 20m traps on each side, you should be able to optimise the dimensions of the antenna to get a low VSWR on all three bands without the need for an ATU. However, in certain directions, a half wave dipole on 15m offers a little less gain than three half waves on that band.
If you go for just a 20m trap on each side, you will benefit from the slightly higher gain offered by the three half waves on 15m but you will never get better than about 2:1 VSWR on 15m because a dipole cut for 40m is not exactly resonant on the 15m band. Of course you can match it on 15m by using an ATU.
In practice, there is very little difference between the two options. If you have two traps on each leg, this adds to the weight of the antenna so personally I would go with the single trap on each leg and accept the somewhat poorer VSWR on 15m.
For my SOTA activations I use linked dipoles with no traps, and I have always just used the 40m dipole on 15m. I have never bothered with a separate 15m half-wave section.
Good luck! Try both methods and see which works out better in practice!