Compare stability of transition metal cations in aqueous solution
It is written in NCERT Chemistry class 12 "In a medium like water, d3 is more stable than d5 (see CFSE). 

CFSE stands for Crystal Field Stabilisation Energy. Basically in water there are h2o molecules which complexate the ions. Assuming octahedral geometry the d orbitals split into d3 (lower in energy compared to barycentre) and d2 (higher in energy compared to barycentre). If you only fill the d3 then you would actually have a more stable configuration as compared to d5 situation where the e.g. levels would also be occupied.

This is a more prominent factor than half filled orbital stability. In $\ce{Co^3+}$ there are no stabilizations involved and hence it is least stable.

$\ce{Sc^3+}$ has a noble gas configuration and this dominates all effects in terms of stabilizing the ion.