I think this may prove it :
Let the sum of angles ABC and BCD = 240°
And let the bisectors of angles ABC and BCD be as shown
Since AB = BC = CD
Then by SAS , triangle ABE is congruent to triangle CBE
Similarly, by SAS, triangle CBE is congruent to triangle CDE
So....triangle CDE is congruent to triangle ABE is congruent to triangle CBE
So angles DEC, AEB and BEC are congruent
But angles ABC and BCD sum to 240
So 1/2 ABC + 1/2 BCD = 120
But 1/2ABC = EBC
And 1/2 of BCD = BCE
So angles EBC and BCE = 120
So angle BEC = 180 - (m EBC + BCE) = 180 - 120 = 60
But BEC = AEB = DEC.....so....
60 + 60 + 60 = 180
Therefore angle AED must be straight a straight angle because angle AEB + angle BEC + angle DEC = 60 + 60 + 60 = 180
Thus....E must lie on AD.......and E is the intersection of the bisectors of angles ABC and BCD