Definitely you and your other sister should get a good lawyer now, with expertise in wills and estates. She may need to be told she has to buy you both out of your thirds in the estate and that may prod her to agree to selling.
One thing is not clear to me -- you say that your sister has moved very slowly (or not at all) on things like choosing items, cleaning out the house, etc. And then you say that "if two out of three agree to sell the house, is there any way to force the third one to sell?" But you never come straight out and say, "Sister 1 and I want to sell and have told Sister 2 that but Sister 2 has clearly said she does not want to sell."
Do you see the question? Has your sister said flat-out and clearly, "I do not want to sell this house" or is your issue more one of, "If and when we ask her I think she's going to balk, so what's the possible scenario IF she says no"?
The answer matters. If you and Sis 1 have not sat down, in person, with Sis 2 and said point-blank, "We both now want to sell the house, period; what is your position on this, definitively?" then you don't know for sure that she WILL balk. She might surprise you and say, OK, let's get this over with. In fact -- she might be more willing to say yes to sale if you and Sis 1 just swallow the fact that Sis 2 has been a pain and offer: "We want to sell the house. If you will say yes and sign off legally on the sale, after you do so, we will ensure that the house gets cleaned out etc. We would prefer that you do it but we are ready to sell this property after three years."
Not fair, I know. She should do whatever she's supposed to do. But you can't change people and can only change how you react to them. She is not going to alter her foot-dragging ways, so offering to clear it all out and hand her her share of furniture etc. may be the fastest way to get the property off your hands now and not in a few more years when it's got even more maintenance YOU will have to pay for. But do nothing without her legally signing off on a sale.
That's why you absolutely need an attorney. Tell her the lawyer is there to protect ALL three of you, not just you and Sis 1.
By the way, I do not at all think you are being cold! You are being realistic. Your father would not want you and your sisters burdened with a piece of property that was costing you money, and it's too bad that the house has already eaten the money he did leave -- that was wasted, unfortunately, and Sis 2 needs to be made (by the attorney) to see that.
It is not some black mark against you to need or even want to sell your childhood home. Is Sis 2 either subtly or blatantly accusing you and Sis 1 of being cold and rejecting your beloved home, memories, etc.? Does she live closer to it and therefore feels very attached, or has to drive past it more often, or is she overall having a tough time dealing with this ending chapter to your mutual childhoods? Does she perhaps feel that once the house is sold, her youth is gone irrevocably? (Yes, I've known folks who went that route and it was hell for their relatives.) I would bear all that in mind, but it is not a reason to give in to any emotional blackmail to keep a house with your names on it when it stands there empty. Your father would not want an empty house to become a money pit or a source of anger among you.
We sold my mom's house in six months after she died. I hated to do it but she was no longer there to visit. I was glad it sold so very quickly so that we could get on with healing in other ways--I would have kept visiting and feeling blue if we had lingered on a sale. And my mother was clear that it was ours to do with exactly as we pleased, preferably to sell since no one else lived in her town. I was fortunate that my one sibling was totally on board with selling and was a big help in many ways.