Relle,
I definitely found my third pregnancy incredibly tiring and uncomfortable. I worked (part time) up until 4 weeks before Lucy was born but I was just exhausted.
With Eva (4th) I found the pregnancy incredibly uncomfortable too. I was HUGE and tired. However, I bounced back fine.
You will find just the added physical strains of caring for two other little people really tough. I made sure if the kids were stationary I was sitting! If they were napping so was I! The bathing was a nightmare so they all showered with me in the end!
You will get through it's just a little bit tougher.