Instead of using a categorized axis, I'd recommend using a numerical axis and assigning the appropriate categories using the labels.formatter() function callback.

yAxis: [{
    tickPositions: [0, 1, 2],
    labels: {
        formatter: function () {
            const categories = ["Low", "Medium", "High"];
            return categories[this.value];
        }
    }
}]

Demo: https://jsfiddle.net/BlackLabel/58xwtcaj/

API: https://api.highcharts.com/highcharts/yAxis.labels.formatter