Q20) If I have a bread that the baker made for me special with 3 grains (meals) in the same quantities: oat, rice and buckwheat. What would the beracha rishona and achrona be on such an item? Would I need to eat 3 kazayit of bread to ensure I ate 1 kazayit of oat flour or would 1 kazayit be enough to merit a birkat hamazon?
A20) Assuming the oat four was added for more than just a binding ingredient - the bracha rishona would be ha'motzei. (See Ikar and Tofel for more about this binding ingredint stuff)
Concerning the bracha achrona - Let's take a look at the major halachic opinions:
The GRA, or vilna Gaon, held that a kazayit is determined solely based on the amount of flour in an item.
On the opposite extreme, the Pri Chadash explains that once an item contains some flour the entire unit is considered mezonot and one kazayit of cake would require al hamichya even if in the one kazayit piece there is much less than a kazayit of flour.
The Mishna Berura (208:48) says the minhag Olam is like this opinion of the Pri Chadash, yet maintains that ideally one should not rely on this opinion. Likewise, the Igros Moshe (Orach Chaim 1:71) and the psak of Harav Elyashiv is that one should ideally eat a full kazayis of flour.
How far does this extend?
It would seem that when they say only flour counts they mean even the water or sugar which are fully integrated into a single unit - EVEN they don't contribute to the kazayit's volume.
However, Rav Shlomo Zalman Aurbach (Teshuvot Minchat Shlomo 1:91:4) rules that sugar [and it seems to me that it would apply to water as well] do count if fully integrated. However, the cheese in a cheesecake doesn't become integral to the crust and therefore doesn't contribute to the kazayis volume.
As for the Final Halacha (for Ashkenazim): It is certainly preferable to try and count only the flour in determining the volume of a kazayit. However, even within this approach one can rely on R' Aurbach that it doesn't mean specifically just flour. Rather, one may include ingredients integrated within the item such as sugar, other spices, oil, butter and water. On the other hand if there are raisin scattered around - it would be best to not count them. Therefore, the best advice is to be safe and eat a little more than a kazayit to avoid an possible issue.
The Final Halacha for Sephardim:
In between the two extremes of the Pri Chadash and Vilna Goan is a third compromise opinion - the Ginat Veradim. He explains that when an item consists of 1/6th flour - then we treat the entire piece as mezonot. Therefore, if one eats 1 kazayit of cake whose batter had at least 17% flour - should say al hamichya.
This is the psak of Hacham Ben Sion Aba Shaul (Or LeSion 2, pg. 169) as well as Rabbi Moshe Halevi (Birkat Hashem 2, pg. 166) as well as Harav Ovadya Yosef (Hazon Ovadya: Berachot, pg. 190) and Rabbi Eli Mansour of the Syrian Community in Brooklyn NY