Having lost both my parents at a young age (8 for dad, 9 for mum) all i can say is i feel for them, as it will be very hard for them all to comprehend.
i think the best thing you can do to help with the grieving process is to allow them to talk to you about their mum, and let them know that its ok to remember her or to even forget somethings ( iknow i cant remember a lot about my parents). Also, encourage them to look at photos to keep what ever memories they may have. What they need is someone to love them all at the same time and i am so happy that you are willing to do that. It isnt easy but i can guarantee that if you cuddle them all at once and give them all equal attn, they will begin to see (it may take some time) that you love them all equally.
I truly hope things go well for you with such a tender situation. Love to you and the girls

Wow, what a privilege you have to be there for those three children. Of course they will fight for your attention. In a way they have lost everything and have to start again. It is good to do things with them together. But also try to have quality time with each of them on their own - perhaps while the other two are watching TV or something. It is especially important that the eight-year-old feels special right now. She must feel so pushed out from her place as the oldest. Maybe you are not the primary carer but what you are doing for these three can make a big difference in their shattered lives. Keep up the good work!

How wonderful for you to extend your hand to them.  Have you talked to the grandparents to see what they would like as far as help?   Maybe they have something in mind that you could do, like watching them during the day or going to buy the food once in a while or even donate clothing.  Let them know that you want to help them out and anything you can do, offer it to them.  Best of luck and hope all goes well.