For a Special Facility in a Bastion that given the order to Craft, can you assign multiple hirelings to craft that item to reduce the time to finish crafting? 

Example: A Workshop has 3 hirelings and can Craft using the PHB rules based on the tools available. Lets keep it simple and say Woodcarver's Tools are available and we just follow the example giving in the PHB with the Heavy Crossbow. If a singular person crafts the weapon, it takes 25gp and 5 days. If 2 people craft, it takes 3 (rounding up for fractions).

Can I have 2 or even all 3 hirelings work on that heavy crossbow? Or do all 3 hirelings work on it, regardless, meaning it would be done in 2 days?

If I'm not mistaken, Facilities with the Craft order have a fixed number of hirelings, except for the Workshop, where you start with 3 hirelings, but you can expand the Facility to gain more.

For example, the Smithy has 2 hirelings, while the Sanctuary has only 1. Depending on the specific Facility, the Craft options vary in time and money, or they simply refer you to use the Crafting rules from the PHB or DMG.

According to that, I'd say the crafting time, if we're following the general Crafting rules, should depend on how many hirelings are working on the item. So, if you're using the Workshop, it should work as you were suggesting in your post.

Personally, I think DMs should have room to modify how Bastion works. For example, they could allow adding more hirelings to Facilities with a fixed number.

Also, the article Help Your Players Get Crafty With the 2024 Dungeon Master's Guide | Dungeons & Dragons includes helpful information:

For crafting in general:

A Little Help From Your Friends

Crafting magic items is a time-intensive process. However, this time can be reduced by asking for a little help. When crafting an item, you can enlist the aid of an assistant, which can be either another character or a hireling from your Bastion, as long as they also have the necessary proficiencies.

The time needed to craft the item is divided between the number of characters or hirelings working on it, and while normally only a single character can assist at a time, your DM might allow more assistants to help out in certain circumstances.

For crafting using Bastions:

If a character is in possession of a Bastion, some special facilities come with the option to issue the Craft order. When this order is issued to a special facility on a Bastion turn, its hireling can undertake crafting an item specific to that facility.

For example, the Arcane Study special facility can be issued the Craft: Magic Item (Arcana) order, provided the character who owns that Bastion is level 9 or higher. Once issued, the hirelings of that facility craft a Common or Uncommon magic item in the same way that a player character could, using the rules from the 2024 Dungeon Master’s Guide. (Magic items that allow the user to cast spells must be crafted by the character, but the facility’s hirelings can assist, thereby halving the amount of time required!)