Look at my posts about processing about 200 kilo plums somewhere on this forum.
Plums will gain about 60% in juice. So for 3 lbs that will be not an awfull lot.
When underripe they will be very low in sugar (mine were at 1055 this year and they were very ripe)
and high in acid (mine were at 10 gram per liter), underripe will be higher.
So either you will have to dillute the juice a lot which gives no flavor left or blend it in with a very low acid juice like banana juice, or a flower extract like dandelions or elderflowers.
Let them ripe more, then wash and then freeze until you have more.
Or make jam and pie.
Luc