As Vonner stated, dont get a 3 channel, or a 5 channel, you will regret it. Not worth the cost savings at all. However, if you insist on taking the least expensive route, use Basshead760's idea. Although not particularly ideal, it will work much better than a 3 or 5 channel. I did this a few years back in my ex's 05 single cab. Ch 1 and 2 for the front door speakers, and ch 3 and 4 bridged mono to the sub. Keep in mind that this will ONLY work if you have a 4 ch amp with individual low pass and high pass filtering for ch 1/2 and 3/4. Some cheaper amps will not have this option, just keep that in mind before you purchase.
With the above in mind, I HIGHLY recommend an amp for your door speakers! They are JL speakers which is an excellent brand (one of my favorite) and although they require minimal wattage compared to some other brands, a typical 50x4w deck is not near enough to produce CLEAN and CLEAR sound quality from those speakers. Yes, they will work fine without an amp, but will be more likely to blow a voice coil from lack of power (yes, a speaker can and will "blow" from being underpowered)
I have installed sound systems in vehicles for years, and am also MECP (Mobile Electronics Certified Pro.) and my .02 would be that if you are going to run a product like JL audio (even though they are just coax and not components does not make a difference) do yourself, your truck, your speakers, and everyone who rides in your truck a favor... get a good quality 4 ch amp for your highs and a 1 or 2 ch for your sub. A system is not complete without 2 amps. I wouldnt be stressing the important of this so much if i knew it really didnt matter. I have installed cheap crap that didtnt even have a brand logo on it to $20,000+ systems in lamborghini's, range rovers, bently's, you name it... and let me tell you this, in my 6 years of doing this, I can personally guarantee you that using decent amplifiers AND the right size power/ground cable will drastically improve ANY systems sound quality and capabilities as well as increase the life of your products. I would say take it to a small shop and see what they say. You dont have to purchase, but I bet they will tell you the same thing.
Let me know if you have any questions about this stuff. I am quite knowledgeable in this area and enjoy helping with electrical and sound system design/product questions. Good luck
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