OOC: First, you are walking with the pack mules, not riding, so you have no penalty from being on horseback. Second, I will allow melee fighters who hold position to ignore initiative during the first round and get the first attack when the creatures come into range. After that, combat will flow by order of initiative.
Third, you will roll a d20 to hit. Add your character class BonusToHit with your Dex bonus (if using ranged) or Str bonus (if using melee) to your attacks. Zephia is a rogue, so she has a +1 BtH at level 3, and she is using a ranged weapon so she adds a +2 to her attack from her Dex bonus, or a total of +3. She gets no Str bonus, so her total melee attack bonus would only by +1 from the class BtH.
Bella has a class BtH of +3 for being a fighter. She also gets a +2 to that with melee weapons due to her Strength. She specializes in longswords, so she gets an additional +1 when using one. If using a ranged weapon, her lower Dex bonus only gives her a +1. So her total BtH would be a +6 with a longsword, +5 with any other melee weapon, and +4 with a ranged weapon (said crossbow).
It looks like Briony is waiting for melee engagement with her two-handed sword. In the coming melee round she will get a +3 for character class BtH at her level, +2 for Str bonus, and +1 for weapon specialization, or a +6 total BtH for her two-handed sword, +5 for a different melee weapon, or due to her +1 bonus for Dex, she would need a +4 for a ranged weapon.