Whilst the three species of oat-grass are readily identifiable as a group separating them is, to my eye anyway, more difficult. This is a case of taking other factors in to account to try and come to a conclusion. Meadow oat-grass (Helictotrichon pratense) is a medium to tall grass species and where it occurs it can be abundant and the dominant vegetation.
How do you distinguish from the downy and yellow oat-grasses? Firstly, meadow oat-grass has glaucous leaves and sheath; this blueish colouring rather than bright green is a helpful indicator, so too the fact the leaves and sheath are smooth and without hairs. The downy and yellow varieties have a distinct preference for lime soil whereas meadow oat-grass grows in neutral soils. Finally, meadow oat-grass flowers a little later than its cousins, the spikes appearing in July and early August whereas the other two flower in May and June, possibly early July.
Not easy but with practice it can be done, I still need a lot of practice on these three!