I thought the specs given for these indicators/LEDs was strange.
The specs say they can be powered with anything from 3V to 9V and that the current is 20mA.
The only way I can think of that this could be achieved would be if the indicators contained a constant-current circuit that adjusts to the input voltage so that the current is always kept at 20mA.
But a circuit like that would be unusual and make the indicators quite expensive.
So I suspect the specification given is incomplete, and only a simple current-limiting resistor is built in to the indicators.
As a result, the 20mA current figure is probably only seen when a 9V supply is used. When the supply voltage is only 3V, the current will be much less than 20mA and the indicator will be much less bright because of that.
But in theory, you could test an indicator with 3V supply, and if you measure the current and find it to be less than 20mA, you could request a refund from the seller, due to failure to meet the 20mA spec. 