He concealed the fact that he had divorced his wife thrice (in writing) in a talaaq form, then his wife found out about it. Does the talaaq count as such?

Praise be to Allah.

Firstly: 

Talaaq (divorce) may be done in writing, and counts as such if it is accompanied by the intervention to issue a talaaq. If you do not have the intention of divorce now, and you only wrote it down and are waiting for a suitable time to issue the talaaq, then no talaaq has taken place. 

If you did indeed have the intention of divorce at the time of writing, but you were waiting for a suitable time to inform your wife of that, and not a suitable time for issuing the talaaq, then the talaaq has taken place. 

See the answer to question no. 72291 

Secondly: 

The threefold talaaq counts as one according to the correct scholarly opinion, whether it was done in writing or verbally. See the answer to question no. 96194

And Allah knows best.