${\mathrm{BeH}}_2<{\mathrm{CaH}}_2<{\mathrm{BaH}}_2$

The size of the elements follow the order: $\mathrm{Be}<\mathrm{Ca}<\mathrm{Ba}$

Smaller the size of cation, more will be its polarizing power. Thus, a higher polarization will lead to a greater covalent character, or the least ionic character. Hence, ${\mathrm{BeH}}_2$ will be least ionic. Top to bottom size of cation increase so covalent character decrease and ionic character increases.