Read-Only properties

If you want a “read-only” stored property, use private(set):

private(set) var isEquilateral = false

If it is a property calculated from other properties, then, yes, use computed property:

var isEquilateral: Bool {
    return a == b && b == c
}

For the sake of completeness, and probably needless to say, if it is a constant, you’d just use let:

let isEquilateral = true

Or

struct Triangle {
    let a: Double
    let b: Double
    let c: Double

    let isEquilateral: Bool

    init(a: Double, b: Double, c: Double) {
        self.a = a
        self.b = b
        self.c = c

        isEquilateral = (a == b) && (b == c)
    }
}