I spoke with a local glass retailer and he said instead of using double-pane gas filled windows, I could instead use a 3-layer sandwich of, say, 1/8", 1/4", 1/8" panes to achieve the same efficiency. Did I hear that right? Is there any truth to this? If so, can someone point me to the details on this?
Windows are a huge pile of tradeoffs and conflicting constraints.
More layers of glass trap more layers of air for more insulation - true. Is it the same, more, or less insulation than a particular gas-filled sealed double-pane unit? You have to compare the specific pair, because both vary.
More layers of glass also block more light (glass does not transmit 100% of light - if it transmits 90% (which is typical, but also can vary with specific cases), then two layers is 81% and three is 73% (and 4 would be 65%)
The gas fill in sealed gas filled windows offers an advantage on the insulation front without increasing light loss. It also is a thing that can leak out over time and as the seals fail, and the sealed unit nature becomes a problem when the seals fail and condensed water is trapped in the unit.
Coatings ("low-E") on the glass can affect both the light transmission and the heat loss. Specific coatings may be more effective at trapping interior heat or at preventing exterior solar heat gain, and which you want will vary with how much of your utility load is spent on heating or cooling, though which is easily/affordably available in your local market may not always reflect that as you would hope. i.e. I'm in a primary heating climate and was looking at triple the price to get windows not optimized for air conditioning. Coatings also affect the light transmission and color. Many coatings are only available on sealed units, where they are protected on the inside of the unit.
So, to compare windows, you really need to get specific - U or R value (U is 1/R, so a low U is a high R, and "high R" for a window is so low that most window makers prefer to confuse the average consumer by not providing the R value), light transmission percentage, and Solar Heat Gain Coefficient (SGHC) are probably the major points to compare.
There is a considerable variation in individual examples of triple-glazed and gas-filled double-pane windows, such that you can't generalize to a meaningful extent. There are both good and terrible examples of both types. Likewise, there are gas-filled triple-glazed units and non-filled double-glazed units available.
There are a couple of alternatives, but they aren't as readily available
The good news is that there are energy-efficient alternatives to pure "gas in glass" double/triple pane technology; however, they aren't quite as available on the North American residential market as conventional "gas in glass" IGUs are.
The more established alternative is suspended-film IGU technology, with either an air (LiteView) or gas (Alpen HPP) fill. These IGUs consist of two outer glass panes, with one or more coated suspended films suspended between the two panes to break up the interior into multiple chambers. As a high-performance alternative, they offer weight and profile advantages over multiple-pane units, and also have a long track record of success, with Alpen having used them for 40 years in commercial work, and making them available in a variety of fiberglass frames (either GFR-uPVC or standard GFR-polyester) for residential applications. LiteView seems to be newer on the market, but offers an intriguing combination of air fill and fiberglass spacers that have the potential for an even longer lifespan yet.
The upstart choice is what is known as a vacuum IGU, or VIG, unit. These have only been commercialized quite recently (as of the past decade to decade and a half), with Pilkington offering their Spacia product for architectural and retrofit applications, and VIG Technologies and LandGlass working with Vitro Glazing (aka PPG Glass) to bring the LandVac tempered VIG technology to the US market. The upshot of this is that you get a very thin unit (Spacia units are scarcely thicker than standard single pane glass) with double-pane performance. However, availability is an issue, as I don't know of any residential window manufacturers in North America employing VIG technology. If you're getting totally custom architectural windows made for you, you might be able to get your glazier to incorporate Spacia, but that's not going to be cheap!