Avoiding the "wrong" calamity
You have to assume that the third card will be a calamity: the rules are designed to prevent you being certain, but it's always wise to expect the worst, and very likely in your position.

You need to consider 

Which calamity will it be?  Depending on how many cities your trading partner has, there are several he may have picked up and wish to trade; if you are careful to specify three cards, you won't get more than one, which you may be able to survive.
And will the situation be any better next turn? If this is 'the most optimal trade possible', you really have either to accept it or resign yourself to a game without trading, which isn't likely to end well.

There may be other reasons to reject this specific trade: card-counting indicates that the two calamities you are worried about definitely came out this turn, or other people have decided not to trade with this person already.  But both are fallible; assuming you can actually use the trade cards for a Civilization card, I would suggest you accept.