When booking airline tickets do I need to purchase seats?

If you want to sit together you need to pay the extra to choose your own seats.

MAYBE

Just buying 3 tickets at the same time does not GUARANTEE anything.

If you want to sit side by side then pay the price to do so. SIMPLE

When you decide to fly as cheap as possible you do not get to pick your row mates.

You MIGHT be together but probably not.

There is no guarantee that you will get seats together if you don't pay for them. It is pretty unpredictable though - it will depend on how full the flight is, how many other passengers did choose to purchase their seats, how early you check in, etc.  

Without selecting seats (at your own expense), there is NO guarantee that you will be seated together.

If you don’t select seats at booking (at your own expense), there is NO guarantee that you will be seated together. Theoretically, if one or more of your passengers is under 18, you should be seated however, Spirit and other discount airlines make it VERY clear that if you don't pre-select seats (at your own expense), there is no guarantee or obligation for the airline to seat you together. Yes, it's a gamble and if you win, you’ll save some money but if you lose, you’ll have absolutely NO recourse. This is disclosed to you when you booked your ticket. It has been on the news numerous times. There's literally no excuse for people to feel butt-hurt. Plus, nowadays, most flights run 90% full so the odds of a row of three seats being available on departure day is pretty rare.

And this is not just Spirit. Nearly every airline nowadays charge to pre-select your seat on the base fares.

And for the airlines that make the seat map public, it's important to remember that the seat map is not an accurate reflection of actual seat availability.  A flight might look wide open but then you have to remember the 60 people like you who rolled the dice and didn't pay to pre-select.  Most airlines will assign seats based on who booked their tickets first. 

The cheap fares from some discount airlines charge for seat assignment unless you accept what they assign at check-in from the remaining unassigned seats. They will not guarantee them together or even in the same area of the plane. Ability to change those seats will be limited. If you want to sit together, fork up the money for assigned seats or you should use an airline that includes it. Most airlines have economy and premium seats.

A personal item is what fits under the seat in front of you. Carry-on and checked baggage is extra at Spirit. Everything is a la carte.

Check in early, then you should have no trouble getting seats together.

that is purchasing seats when you do that, i would just tell them you want to sit together