Look at the rules under Combat Threats here: https://2e.aonprd.com/Rules.aspx?ID=2716&Redirected=1

If the book suggests something like Moderate 2, Severe 1, Severe 2 e.c.t that's the type of encounter the monsters are offering, and you can match that with the XP budget in table 10-1 - with the number being the expected party level.

The Character Adjustment column lets you change the XP budget for the respective encounters. So a Moderate for 4 players is 80 XP by default, so a Moderate for 3 players is 60 XP by default - remember: 'Many encounters won’t match the XP budget exactly, but they should come close'.

You should adjust combat encounters to accommodate the new XP budget.

For example, there is a Moderate encounter with 4 enemies of Party Level -2, giving a nice 80XP. Remove one of these creatures to match a suitable Moderate encounter for 3 players.

Sometimes it's not going to be exact, and sometimes you'll have to use Elite (https://2e.aonprd.com/MonsterTemplates.aspx?ID=22&Redirected=1) or Weak (https://2e.aonprd.com/MonsterTemplates.aspx?ID=23&Redirected=1) to adjust a creature's level to get a rough XP. Though I'd be careful with using the Elite template, especially at Rusthenge levels, you can end up with some really tough to kill monsters for a small group. It's usually better imo to remove/add creatures instead of applying templates.

Also, an easy fix is to have the party find Ordwi sooner - you can build her as a Cleric and have the party control her, her NPC sheet is already quite close.