It could simply be that you have experienced and been through the 2 pregnancies before. Because of that the excitement and the unknown is not there any more.
The other reason, could be that you are too tired, too run down, may be anaemic and may have slight depression.
Make an appointment to see your doctor if you do not see him routinely to check your pregnancy and ask if you can have a blood test done to check your iron levels.
In the mean time, make sure that you are getting plenty of rest. When your young children are sleeping in the day or out at school, have a nap in the day. Make sure that you eat a healthy and well balanced diet with calcium rich foods - cheese, milk, to protect your teeth and bones and to have the right minerals and vitamins for your baby to have strong bones.
Make an appointment to see your doctor and get checked out for any signs of anaemia.
Wishing you all the best.
It might be just as simple as, been there, done that. Most mothers of many children can easily remember things about the first baby, and took more pics, and wrote stuff down, and by the 3rd baby you've kind of done it all before.