I make up a linked dipole for 40m and 20m. Then, I add small air cored 1.1 microHenry inductors at the links to make it resonate on the 15m band, as a 3/2 wavelength antenna. This is a great solution for homebrew or kit rigs that do not have an inbuilt tuner and need a naturally resonant antenna on all bands.
SOTABeams post on modelling and options for getting a 40m dipole to resonate on 15m:
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