How 3 phase unbalanced current flow through neutral?

Kirchoff's Current Law states that the signed sum of the currents entering a node is zero. If the neutral point is the node, then, in a balanced system, hot matches the other two hots, resulting in no current through neutral. Any imbalance, either due to neutral to hot current flow that is not balanced by the other two neutral to hot current flows, or due to ground fault, will result in a current flow on neutral, so that the sum of zero is maintained.

When you think about this, remember that the law said "signed" and "entering". When you analyze a circuit, simply be consistent in your usage. For instance, in a balanced system, current entering the neutral node from one hot side is considered positive, and the current entering the neutral node from the other hot side is considered negative, i.e. it is leaving, not entering.

This gets more complicated in three phase power, because now you have to consider phase angle, but the concept is exactly the same...

If you are connected in wye, with a neutral, then the neutral conductor will have zero current on it only if the three phase hots have the same current on each. If you do vector analysis on this, adding up sin(x), sin(x+120), and sin(x+240), you get zero.

The same thing happens when you are delta connected, without a neutral, but then the imbalance occurs out in the distribution system, beyond the service Transformers, because the distribution system is generally a wye system.

Ground fault will, of course, "change the rules", because you no longer have only four paths to that neutral point node. In fact, that is how ground fault current interrupters (GFCI's) work - they measure outbound current and compare it to inbound current - they must be equal and opposite, i.e. they cancel each other out - otherwise there is another path - a ground fault - and the device trips.