A woman on my street who had 4 kids had the same problems as you - honestly, she put a kiddie pool out in the yard and sat in it herself! Put the funnels and colanders with in there and let her play with stuff, making waterfalls and so on. Put out a sprinkler and have both of you get wet! Go to a pond or lake and put your beach chair right in the water, and sit while she plays in the shallow water.
School's out, so maybe you can hire a mother's helper for a few hours every day - a 10 or 11 year old is mature enough to supervise a 3 year old while you are home for any major emergency, and has the energy to run around or take a nature walk (take a bucket and have them collect something interesting like rocks and leaves. Then bring the rocks home, wash them, and paint them as paperweights or decorations - put them on the porch or as a border around the mailbox, or give to relatives at the holidays - no kidding!)
Try not to feel guilty - I know it's hard. But you're more tired during this pregnancy because this time you have a child to care for during the day! But this is good practice for not having the flexibility to be with your older child all the time. Let her get used to being care for or entertained by someone else, and it may even make the adjustment easier when the baby comes.
Good luck!