Can I get 3 double pole breakers in my circuit breaker?
I think you're going to need an electric service upgrade if you want to use this water heater.

Your 100 amp service is already somewhat loaded, as well as the box being physically full.

If you call for hot water while the dryer or range is in use (as obvious heavy loads on your panel), you may well trip the main, if you do not upgrade to 200 amp service before installing this water heater.

A different water heater might make more sense, unless you have other reasons to upgrade your electric service; one of the heat-pump water-heaters (or a heat-pump-add-on to an existing water heater tank), perhaps.

Edit, Add: Unless this is a house that stands vacant for long periods of time, there's surprisingly little "standby loss" (what you save by using an instant tankless electric resistance heater .vs. a tank-type electric resistance heater that's insulated to modern standards) - and if it does stand vacant for long periods, you can switch the water heater off when you leave. Changing to a heat pump water heater (HPWH) can, by contrast, save quite a bit and get you "free dehumidificaton" in the summertime. If you have gas available, it's more of a toss-up - gas and HPWH can be similar in cost to run, and a gas water heater will usually cost considerably less to buy. Most of the cost of running a normal (modern, well-insulated) tank-type water heater is, in fact, the cost of heating the water that you actually use. Most households will see little savings from using an instant-tankless waterheater, in practice.