I just made a Ranger 6/Druid 3. I am getting the correct amount of spell slots (4 1st, 3 2nd, 3 3rd) but I cannot prepare 3rd level spells.

Prepared Spells of Level 1+. You prepare the list of level 1+ spells that are available for you to cast with this feature. ...

The number of spells on your list increases as you gain Druid levels, as shown in the Prepared Spells column of the Druid Features table. Whenever that number increases, choose additional spells from the Druid spell list until the number of spells on your list matches the number on the table. The chosen spells must be of a level for which you have spell slots. For example, if you’re a level 3 Druid, your list of prepared spells can include six spells of levels 1 and 2 in any combination.

If another Druid feature gives you spells that you always have prepared, those spells don’t count against the number of spells you can prepare with this feature, but those spells otherwise count as Druid spells for you.

Changing Your Prepared Spells. Whenever you finish a Long Rest, you can change your list of prepared spells, replacing any of the spells with other Druid spells for which you have spell slots.

Prepared Spells of Level 1+. You prepare the list of level 1+ spells that are available for you to cast with this feature. ...

The number of spells on your list increases as you gain Ranger levels, as shown in the Prepared Spells column of the Ranger Features table. Whenever that number increases, choose additional Ranger spells until the number of spells on your list matches the number in the Ranger Features table. The chosen spells must be of a level for which you have spell slots. For example, if you’re a level 5 Ranger, your list of prepared spells can include six Ranger spells of level 1 or 2 in any combination.

If another Ranger feature gives you spells that you always have prepared, those spells don’t count against the number of spells you can prepare with this feature, but those spells otherwise count as Ranger spells for you.

Changing Your Prepared Spells. Whenever you finish a Long Rest, you can replace one spell on your list with another Ranger spell for which you have spell slots.

So my Ranger 6/Druid 3 should be able to know 3rd level Ranger spells and prepare 3rd level Druid spells, but DnDBeyond does not allow for that.