I am going to spend a couple days on Lanai, and need to leave our rental car somewhere hear the ferry on Maui for 3 days. Does anyone know if the local parking lots in Lahaina have daily rates that allow fir multiple days,and if so, how much that might be?

Are you staying at a hotel in Lahaina before or after you go on to Lanai? That might work.

I have no current useful information as I seldom visit Lahaina proper

I am curious where you are staying as uncle Larry has shut down all hotels for reno

I don't think those lots offer long-term parking rates. As far as I know they don't want the lots used for that purpose.

You could give up your rental for the 3 days, then just get another rental. The agencies will pick you up at the ferry dock. It's a bit of a hassle, but quite workable.
One time, we flew from HNL to Lanai, stayed 3 nights, took the ferry to Lahaina, got picked up at the dock and got a car there, and spent a week on Maui, dropping the car at OGG. Not the same as you, but similar, and relatively painless!
I was quite impressed at how simple it was to go from Lanai to Maui by ferry as opposed to air. Especially if you pack light. We had golf clubs, and even that wasn't bad.

Couldn't you call or email the ferry office and find out? (The lot we used many years ago is no longer available for overnight.)