JavaScript (ES6), 28 bytes

(a,b,c)=>c-2*b+a?c*c/b:2*c-b

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JavaScript (ES6), 28 bytes

(a,b,c)=>c-2*b+a?c*c/b:2*c-b

Try it online!

Commented

(a, b, c) => // given the 3 integers,
c - 2 * b + a ? // if the sequence is not arithmetic:
 c * c / b // assume it's geometric and return the next geometric term
: // else:
 2 * c - b // return the next arithmetic term