What I said in the title above seems to be roughly true in the European languages that I have checked, so my question is: Could ancient Indo-European, or a precursor of it, used a suffix that meant "add one" after the root words for 2, 4 and 6 to denote 3, 5, and 7 respectively? I mean, maybe three was "two and one more", five was "four and one more", seven was "six and one more". This could explain the similarity in initial letters for these numbers. Thanks.
These words show no signs of sharing a common suffix, let alone one that we can identify as meaning "add one." Actually, there is another explanation often used for this kind of thing: sound changes by analogy that cause counting numbers next to each other to change their first sound to be more similar. This is thought to be the reason why "four" starts with "f" in Germanic languages. (discussed in Historical and Comparative Linguistics, by Raimo Anttila)
Apparently, some think this may also be the source of "s" in the "six" word: the reflexes in Indo-European languages point variously to original *w (Armenian), *sw (Celtic), or *s (Italic, Germanic); one idea is that it started out with *w, and then later an "s" was added at the start by analogy with the "seven" word, and the cluster *sw was then simplified to just *s in some branches. (discussed in the "Italic" chapter, by Robert Coleman, of Indo-European Numerals, edited by Jadranka Gvozdanovic) However, another etymology of Armenian վեց vec' derives it from the cluster *sw via an intermediate step *hw. ("Indo-European 'Six'" by Václav Blažek) The internal structure of the PIE word and the history of how it came to mean "six" are also unclear.
But "two" and "three" are just coincidence as far as I know. In fact, "two" and "three" only start with the same letter in English by coincidence; the sound is not the same, and in other Indo-European languages like German or Spanish the spelling reflects this more: (zwei, drei and dos, tres).