While the baby is still young and not guaranteed to sleep through, you can get UA time in your own house. You need to put all 3 kids to bed, and train the older ones to stay there, and then set the table, eat and talk.

I don't think Dr Harley says that a mother (a breastfeeding mother?) must leave her baby if she is unhappy doing so, but he does insists that you spend time alone together. Spend time in the house for now, while you work on getting the baby to sleep through. In six months' time, your wife might not mind leaving her at all, because she won't need frequent feeding and she won't wake up while you're out for the evening.

An alternative is to get a babysitter, but go out for only two hours (alone - not with friends). If going for a meal takes more than two hours, go for a drink or just for a drive to a beautiful spot in the long evenings. Don't go far from home, so that you can get back quickly if the sitter calls. If you wife takes short breaks, she will find it easier to enjoy going out without fretting.

Most people cannot afford enough babysitting to make a substantial dent into the 15+ hours needed per week. The longest you are likely to manage with small kids is 4 hours per week. You will have to find the remaining hours while you stay at home - with the kids in bed, staying put.