Can I use "Pokemon Collector" to choose Pokemon LEGEND cards as part of the 3 Basic Pokemon I search for in my deck?
No, because Pokemon Collector only lets you choose Basic Pokemon. Pokemon LEGEND are considered "Unevolved Pokemon" when in play, but they are not considered "Basic Pokemon".
Source: HeartGold & SoulSilver FAQ (2010-02-11), PUI Rules Team (2010-02-11)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS1_97