Yes HCO3 is the measurement for the bicarbonates (hardness) and CaCO3 the measurement for alkalinity (buffering capacity)
Chemically, hardness is often defined as the sum of polyvalent cation concentrations dissolved in the water. The most common polyvalent cations in fresh water are calcium and magnesium.
Alkalinity is a measure of the acid-neutralizing capacity of water. Alkalinity is usually reported as equivalents of calcium carbonate (CaCO3).
What is the long suffering home-brewer to make of this? It appears to me and please correct me if I am wrong that hardness represents the total dissolved salts in water and that alkalinity is how much our water can resist change (normally acidic change). Naturally there is a symbiotic relationship between the two. As far as I can tell hardness is not really a problem, its the carbonates that are dissolved in the water that reduces our waters ability to resist change thats the problem. (spit ding!) Neutralize or get rid of them! That is why we can boil the water to get rid of some of the carbonates or we can treat our water with an acidic solution (like CRS) to weaken or neutralize the waters ability to resist change.
Rather interestingly I was reading yesterday that Guinness used to add some sour beer to their famous brew to give it that 'Guinness twang' now they use lactic acid. German brewers add lactic acid to Munich malt and add that to reduce the alkalinity of the mash (acid malt) I think this would be an excellent addition because you typically only need about 1-2 percent (20-100g) to really effect the Mash and bring it down to an acceptable level.
If anyone is interested I can put your water data and your target profile data into various software and we can compare the results. I really like the old forum calculator, its simple and gives instant results treating the mash and sparge separately. It has its limitations though in that it does not take the grain bill into consideration and some of the additions are a little excessive in comparison to other estimates. If it works for you though, do it!