Assalaamu alaykum. if one does not know the supplication that is to be said after the third Takbeer in the funeral prayer, then what should he recite? Is the prayer invalid without it?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

There is no specific Duaa (supplication) for the deceased in the funeral prayer; rather, any general supplication is acceptable, such as saying, “Oh Allah! Forgive him, have mercy on him, and pardon his sins.”

Ibn Qudaamah said:

“The obligation is to say the least of supplications because the Prophet said, 'If you pray (the funeral prayer) for the dead, supplicate sincerely for him.' [Abu Dawood] This can be achieved with the least of supplications (i.e. with any supplication), as what is intended is to intercede for the dead and to supplicate for him, so the minimum of this is an obligation, and it is recommended to make Duaa for oneself, one’s parents, and all the Muslims. (Imaam) Ahmad said, 'There is no timed supplication for the dead.’ (Meaning that it does not have to be long.)”

Allah knows best.