JS Array Help

I want to be able to keep a record of the last 3 tweets my system used....

eg: 949271439605190656,950237352001187841,952851700234137601

So each my my js function runs it recieves a new ID... I want array to drop the last one and add the new one so there is only ever 3.

I have this code:

var last3Ids = [0,0,0];

Select allOpen in new window

if (Object.keys(last3Ids).length >= 3) {
	last3Ids = last3Ids.slice(last3Ids.length - 3, 3);
}
last3Ids.push(element.tweet_id);

Select allOpen in new window

But the array goes between having 3 then 4 then 3 then 4 id's

Can someone help why this is happening?