2 If they early, they the tents.

2. Если бы они приехали рано, то поставили бы палатки.

В третьем типе условных предложений главное предложение использует конструкцию would have + past participle, а условное предложение использует past perfect.

Поэтому заполним пропуски:

If they had arrived early, they wouldn't have had to put up the tents.

Ответ: If they had arrived early, they wouldn't have had to put up the tents.