The best way to solve this is not with trigonometry, but with vector math!
I'm going to write some quick pseudocode of what you need to check for difference in angles, if you want the juicy details of the whys and hows scroll further down.
float l1x = entity.getPosition().x - player.getPosition().x;
float l1y = entity.getPosition().y - player.getPosition().y;
float l1z = entity.getPosition().z - player.getPosition().z;
float l1mag = Math.sqrt(l1x*l1x + l1y*l1y + l1z*l1z);
float l2x = Math.sin(turn);
float l2y = 0;
float l2z = Math.cos(turn);
float dot = l1x * l2x + l1y * l2y + l1z * l2z;
float angle = Math.acos( dot / l1mag);
return Math.abs(angle) <= FOV/2;
Copy code at own risk, it was written completly freestyle and I give no guarantees it will actually function straight off the bat.
I'm going to assume you know what a vector is, if not the answer might not make a lot of sense but I'll try to be pedagogical.
Getting the angle of two points is utterly nonsensical, two points are connected with a line, an alone line has no angle, but there is an angle between two lines.
So our first line will be the line through the two points but what will the second line be, we could pick an arbitrary direction but then we'll have to figure out absolute degrees inte relative degrees and as you noticed that's just a mess we want to stay clear of.
Since we're only intrested in where the second object is relative to the players FOV it makes much sense for the second line to always be from where the player is to where the player is looking, this way 0° will always be right in front and 180° will always be behind, much simpler to get our heads around wouldn't you say?
Now we come to the tricky part, how do we know where the player is looking?
The best would be to have a unit vector denoting direction that gets tranformed whenever the player turns, this is something I would highly recommend to implement in the future but for now we'll stick to something relativly more simple using only what you posted.
Assuming 0° means the player is facing +z and he can only turn around the y-axis the equation for figuring out the points position in the world is:
pointX = player.x+sin(player.turn)
pointY = player.y
pointZ = player.z+cos(player.turn)
Notice we didn't multiply the sin or cos with anything, this is because the the point will always be 1 unit in front of the player meaning the line from player to point will always be 1 in length, why 1? Because it's easier to work with.
Now we have two lines, l1 from the player to the object and l2 from the player to where the player is looking, but how do we represent the lines? Should we have three different points? No, we use vectors! Vectors are excellent for representing lines, they contain both direction and magnitude.
So now that we have two seperate vectors there is a very simple way to get the angle between them; we use their dot product! The dot product of two vectors is defined as:
dot = l1.x * l2.x + l1.y * l2.y + l1.z * l2.z
Now you might find yourself asking "What does this have to do with angles? Stop stealing pictures from wikipedia and being useless!" Well it all comes down to the second definition of the dot product namely:
Translated this says that the dot product of two vectors is equal to the product of the two vectors magnitude(length) and the cosine of the angle between them the last part is important becasue it means that if we know the length of the vectors we can figure out the angle with a simple equation.
We know the length of the vector l2 is 1 and to calculate the length of l1 we take square root of the sum of the squares of its components.
l1.mag = sqrt(l1.x^2 + l1.y^2 + l1.z^2
And the pivotal equation that figures out the angle would then be:
angle = acos(dot / l1.mag)
And then we compare the absolute value of the angle to the fov/2.
if( abs(angle) <= FOV) return true