As Lizzy realizes you intend to bathe with her, she protests "Ah..it's not a big shower. It would be awful crowded with 2..no, 3 people in there."

You soothe her fears with a hug and "Well, Lillian is dissolving nicely, so she won't get in the way, and besides, getting close and crowded is the idea of showering together."

"I'm not sure that's such a good idea."

"Come on. You will find a shower much more fun when somebody else soaps you up."

"Ok, but that will be all you will do."

Well, it is all you will do at first.

As she thought, the 2/3/2.5 of you fill up the shower and then some. You have to do a fair amount of wiggling and squeezing to get in and close the door. But that is a good deal of good clear [obviously] fun. And Lizzy soon realizes that she not only can't object to your soaping up sensitive areas, but she doesn't object to your doing a "thoro" job.
There are still objections along the way, but these are largely due to non-sexual pokings. It's all to easy to get an elbow or knee in an irritating spot. But the main problem soon comes down to finding a possible position. [Lillian is a deplorably effective chastity belt.]