Maid...which one to fired?

We have 3 maids who help us to tend the house.
The older one is Bi Mis, who has been with the family for 17 years. Now she's in her fifties. Her job is to do laundries (no machine wash)and trice a week wiping all the cabinets. Also go to wet market to do shopping for my mum and help to look after Megan.
She used to do all the mopping and wiping floor on 2nd and 3thd floor until she complained that the toll was too much and she's too tired so it was reduced to cleaned my mum's bathroom only. She works from 8.00- 14.00

The second one is Ima, she's already 7 years with family, now she's pregnant and will due around hari raya but still she want to continue to work with us and asked mum permission for 3 month leaves.
She used to be stay at home maid until married last year. Her job is on 1st floor, mostly kitchen jobs. she also mopped the floor daily and cleaned the bathroom on 1st floor. Twice a week, she cleaned all the windows on her level floor and trice a week, wiping all the cabinets. She work from 8.00- 15.30

The third one is Ac, She only with us for 1.5 year. Her job is to clean 2 and 3 floor including floors, cabinets, 2 bathrooms and look after Megan. She is quick witted and smart..too smart actually that she become bit lau chua. she work from 8.15-17.00

Now, the problem is none are stay at home maid and we need stay at home maid. Which one to fired? from logic thinking, it should be bi Mis, but since she's been around for so long, mum tend to like her as family and she has gentle heart and very obedient.
The second option is Ima, but she got my mum soft heart because she's good maid, she's clean and smart and she can ward off all the preman2 if they come to ask money and also she's the poorest one, her hubby is only a driver and his salary will not enough to support her and the coming baby.
The only option is Ac, but Ac is smart.. you just point one thing and she'll remember...And the most important is she really adore Megan, since she's childless, and she can play with Megan. Headache arrrrrr.