Allocate one to one time with the first and second one as your first one is only 4. May be after your hubby is back from work you can play with the second one while your hubby can spend time with the first and then switch after some time. Lucky the grand parents are there to take care of the youngest.
Carve out a specific time with your second one. For example, Tuesday afternoons are date time with him and you spend time just with him with no distractions from the others. Let your eldest know that it is date time with second child
Always include the middle child in everything that you do, do not show any preference for the eldest or youngest child. Maybe gently remind your family members as well (:
Involve all 3 in your outings! Very tiring but it will be great bonding for the siblings!
Spend time altogether with all the kids I felt. Always acknowledge them.