Heavy ion injector I-3 represents two-gap 2.5 MHz resonator with accelerating voltage 2x2 MV. It's used with laser ion source for acceleration of heavy ions in wide range of charge to mass ratio. As a result of modernization, injector structure will be supplemented by the second two-gap resonator, rf voltage will be increased to 3x4 MV and accelerated beam structure has to be improved by increasing accelerating frequency to 5 MHz. Design features of upgraded linac and peculiarity of beam dynamics for different types of ions are presented.