Quick answer
A 3.5 core cable has three full-size phase cores and a fourth core, the neutral, of reduced cross-section. The half in the name refers to the smaller neutral, which is commonly about half the phase size. For example, a cable with three 70 sq mm phase cores and a 35 sq mm neutral is described as 3.5 core.
The reduced neutral works because, in a three-phase system with reasonably balanced loads, the neutral carries little current. Where the loads are heavily unbalanced, or where non-linear loads such as drives and switched-mode supplies add harmonic current to the neutral, a full-size four core cable is the safer choice. APAR's LV XLPE power cables are available in 2, 3 and 4 core forms with a reduced neutral conductor.