I would convert the truth table into an K-map:
https://en.wikipedia.org/wiki/Karnaugh_map

Karnaugh is not usuable for your expression. There are 7 input signals 
(x(0) ... x(5) + i) - by far too much for Karnaugh.
If you want to transform the expression into a PLD representation (NOT - 
AND - OR stages) you should look for 'De Morgan's rules'. Also, XOR must 
be translated into a NOT/AND/OR expression first.
It is a lot of paper work to bring it into the right format. I would 
suggest to do this first for an easier expression than this one.
However, you can ignore the science behind and just use the expression 
as is. The PLD compiler will do the work for you.

> y <= i xor ((x(5) and x(4) and x(3)) or (x(2) and x(1) and x(0)));

> I am stuck at the fact that the minimized function
> only has 3 or-terms and 3 different variables, meanwhile the defined
> PLDcell has 7 inputs..

Then you made a mistake in the minimization.  None of the 7 variables 
(i, x(0) to x(5)) can be eliminated.

This is how the PLD logic interprets your formula:

1

y =

2

   (    x(0) and     x(1) and x(2) and not i)

3

or (not x(0) and not x(3)          and     i) 

4

or (not x(0) and not x(4)          and     i) 

5

or (not x(0) and not x(5)          and     i) 

6

or (not x(1) and not x(3)          and     i) 

7

or (not x(1) and not x(4)          and     i) 

8

or (not x(1) and not x(5)          and     i) 

9

or (not x(2) and not x(3)          and     i) 

10

or (not x(2) and not x(4)          and     i) 

11

or (not x(2) and not x(5)          and     i) 

12

or (    x(3) and     x(4) and x(5) and not i)

It is indeed very complex to come here, but you can try it by hand and 
check your result using an online calculator, e.g.
https://www.dcode.fr/boolean-expressions-calculator
(replace x(0) with a, x(1) with b a.s.o.)

I tried doing a truth table for each, the function and the PLDcell, and
try to find where it matches? but it doesn't seem like the correct way
because the PLDcell has 2^7 possible combinations. Happy to read stuff,
if someone can point me in the right direction