If we take a three phase transformer with secondary connected in delta and do not connect load on one phase. Other phases will be loaded to full capacity.
What will be the consequences?
Clearly, circulating-current will flow in the Secondary delta-winding. Primary winding-currents, as well as Supply line-currents will change.
If you want to know the magnitude of the problem please provide the Primary-winding configuration, i.e., Wye or Delta.
if secondary side is delta connected and you put a load on only two phases, it is very clear that, two line output current will divided into three phases (coils) of secondary side winding.
secondary winding current will be unbalanced. Two phase will experience half of the line current and one phase will carry 1.732 * line current.
now you have to calculate, the (1.732 * line current) is higher than the transformer phase current rating of not. If yes then transformer protection need to be operated.
This configuration is equivalent to open delta connection where you can load the transformer up to 57% only.
By using the Open-Delta, or Vee, analogy, you lose the impact of a circulating-current in the Delta-winding.
If primary-winding is Delta-connected, and each load is 1.00 per-unit, then, primary supply current distribution is 1.00 : 1.00 : 1.74
If primary-winding is Wye-connected, neutral status must be known.