I am sure others will advise you on what class pass is needed for what sleeper cabins as they vary with operator.

However I would ask if you have fully considered using sleeper trains rather than reasonable priced hotels or aparthotels?

For 3 people wanting privacy the choices are either a full couchette cabin - basic 4 or 6 bunk beds with no facilities and only a cover sheet so you sleep in your clothes, or a sleeper cabin which will have 3 bunk beds on one side and limited wash facilities. Very few premium cabins with WC are available and the price, even with a pass is much more than a reasonable overnight B&B or hotel such as an Ibis.
Although there are a growing number of night trains in Europe you will still find they only service limited A to Bs and have relatively few places on each train.

In addition catering is, at best, an airline type breakfast, but most have no or limited catering.

Also remember that night trains mean you miss the scenery.

There are many youtube videos on night sleepers.

From FR to IT are NO nighttrains for now-though you could if wanted use one till Nice and continue on along coast. This FR/SNCF train has only couchette, no real sleeper. Most current such trains run as NightJet by Oebb=Austrian rails and obviously thus to/fro that country. The real sleepers are in high demand-or perhaps also in very low nrs available and its a nightmare to get them on the days that most tourists have very fixed itineraries.

Though I am not as astute as yorkie about avoiding them, I share his idea that these trains are a very much overhyped thing lately and the comfort is much lower as what a normal HTL would offer at same price. Eating BF on board: forget it-most trips arr early morning and find a better place at much lower price at station. In general food on board trains is very limited and very high priced.

As Mcadv says the biggest operators of night trains is OBB as Nightjet (NJXXX) and Euronight (ENXXX).

Both of these are bookable at the link below but be sure to add all 3 passengers and select the discount as Eurail/Interrail pass for each. This will give you availability and reservation costs for specific trains and dates.

https://www.nightjet.com/en/ticket-buchen#/home

https://www.nightjet.com/en/buchung/tarifinformationen/interrail-eurail-paesse

It seems a second class pass is adequate for Nightjet, but for example a private 3 bed sleeper will carry a charge of over 220 euro for a journey from Amsterdam to Munich. (Example below is for Nov 2022 on the 1930 departure and arrives at 0711.

In daytime a train from AMS to MUN would cost a fraction of that and even then those requiring reservations would be optional. 

Granted it would take about 7 hours but by staying in an Amsterdam hotel you get the chance to eat out and enjoy the city at night. Alternatively you could leave Amsterdam a little earlier and stay overnight at a suitable town or city halfway to Munich, then resume after breakfast and be in Munich for lunch.

For sleepers in Western Europe you only need a second class pass whatever type of sleeper you book.

@Yorkie €220 for 3 pax? That includes shower facilities in cabin? Is this price with Eurail pass or before the pass discount? 

@cdwatkins19 can we book deluxe sleeper with second class eurail?

For first class day train, what kind of complimentary refreshments are included? Anyone knows? 

That is after discount - Eurail only saves the travel part of the ticket so full price is 395 euro

On sleepers there is no extra for a first class pass holder - see the website for included refreshments. On day tickets it is very variable, but assume a small drink and packet of snacks plus some offer at seat service (mostly to be paid for. Only serious benefi on day trains is space and quality of seat.

Unfortunately the search I have done merely offers a sleeper cabin, but doesn’t distinguish between standard sleeper(washing facilities) and deluxe cabin. However it seems that the class of your pass is irrelevant. I assume a deluxe cabin would catty a premium. Also only a deluxe cabin has a WC.

I still advise staying in a hotel and using day trains.

Does anyone knows what is the percentage of discount if I were to purchase a Eurail for day trains?

There is no special Eurail pass for day trains. The Eurail pass is valid on all trains included in the Eurail offer.

@AnnaB i mean when you purchase Eurail pass what’s the discounted rate for day trains 

There is no discount for a day train pass because there is no day only pass.