1.expand 2.believe 3.given
解析：
1.expand指扩大,增加（尺码,数量等）;extend指伸长,延长(时间)等
2.believe it or not是个惯用法,意为”信不信由你”．
eg.Believe it or not,he asked me to marry him.
3.given介词,意为”考虑到；鉴于”,和后面名词或代词构成介词短语,作状语．句意”考虑到他更多的经验,他本可以做的更好”．本句相当于Considering he had more experience,he woula have done it batter.