He needs to travel another 169.4 km.
How much is left for the 1000km?
The truck driver has traveled a total of 345.7 km + 276.5 km + 208.4 km = 830.6 km.
To travel 1,000 km, the truck driver needs to travel 1,000 km - 830.6 km = 179.4 km more.
So, the answer is 169.4 km.
Here is a simpler way to understand it:
Imagine that the truck driver has a piggy bank. On Monday, he puts 345.7 coins in the piggy bank. On Tuesday, he puts 276.5 coins in the piggy bank. On Wednesday, he puts 208.4 coins in the piggy bank.
To reach 1,000 coins, the truck driver needs to put 169.4 more coins in the piggy bank.