Can I ask where you are? Where I am, the MFRC has been a great resource in finding ways to deal with the struggles of one parent being gone. I've had some success with activity booklets for children on how to cope, ideas on how to survive and some great ways to keep that age busy.
My middle child was and still takes longer to adjust to her dad being gone and I still go back to those booklets. I also, left my job and started at home childcare. I can't say it has been an easy 9 months, but eventually it will be worth it!! I promise you're not the only one struggling, it just takes time to figure it all out and figure out what your child needs to make the adjustment easier.
And yes, the behaviour is normal. for a 3 yr old and for a new older sister!