One bolt is recovered, the PC chooses which one.
At the end of the battle, you can recover half your expended
ammunition by taking a minute to search the battlefield. PHB.146
The weapon properties section of the players handbook states that a character who takes a minute to search the battlefield will recover half their expended ammunition.
Whenever you divide a number in the game, round down if you end up
with a fraction, even if the fraction is one-half or greater. (PHB
p.7)
Reading the two rules in context reveals that the PC would find one bolt.
Three bolts were expended and half of 3 is 1.5, this result is rounded down to 1.
To adjudicate which one coloured bolt is recovered (which half of the expended ammunition), the PC who expended the ammunition will choose (since their character is the one searching, actively attempting to locate the ammunition).
In this case the ranger would choose whether the bolt recovered is the red one, the yellow one, or the blue one.
After searching, the ranger could choose to recover the blue bolt.
The players handbook is a resource that often uses the word "you" to refer to the person who is playing D&D as a particular character.
Throughout this chapter, we use the term character sheet to mean
whatever you use to track your character, whether it's a formal
character sheet (like the one at the end of this book), some form of
digital record, or a piece of notebook paper. PHB 11
The above passage denotes that a player is responsible for tracking and recording aspects of their character on a character sheet.
At the end of the battle, you can recover half your expended
ammunition by taking a minute to search the battlefield. PHB.146
The players handbook again refers to "you" in reference to ammunition which the player will recover and record on their character sheet.
Together this denotes that the player will keep track of and record on their character sheet what ammunition is expended and what ammunition is recovered.
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With regard to the normal flow of the game where a DM describes the environment, the players describes what they want to do, and the DM narrates the results of the adventurers actions.
An example of play that would adhere to the rules:
- DM “The Orc lies dead on the battlefield, murdered by your arrows, there are no other enemies”
- Player “Agar the ranger searches the battlefield for his arrows”
- DM “You recover one arrow”
- Player records the recovered ‘blue arrow’ on his character’s sheet
Since it is not possible to show every permutation of how recovering ammunition could be played out in a game, this example is provided to show one of many which are permitted by the rules and would result in the player choosing which half of their expended ammunition is recovered.