First, Group III is the easiest to devide because 来{く}る and する are the only verbs that belong to it. These verbverbs have each irregular conjugation as you probably know.
Next, I'll tell about verbs that end with ''る''.
You have to care the previous vowel to ''る''.
For example, let's watch a verb ''乗{の}る''. This verb can be written ''noru'' in the alphabet, then the previous vowel to ''る'' is ''o''. If
If the previous vowel is ''a'', ''u'', or ''o'', the verb belongs to Group I.
Memorizing these verbs, you can tell that 走{はし}ります and 走{はし}った are correct, not *走{はし}ます and *走{はし}た, 上食{あた}げべます and 上食{あた}げたべた are correct not *上*食{あた}げりべります and *上*食{あた}げっべった because 上げる食べる isn't in the list above though it ends ''e-る''.