Least
Least is an adjective that means the smallest amount:
Least是一个形容词，意思是“最小得;最少得(little得很可以)”。
We need the least amount of friction possible.
我们需要尽专家少得摩擦。
Lest
Lest is a conjunction that refers to averting or circumventing something
Lest则是一个连词，表示“回避或回避某事;唯恐，以免，只怕”
Memorial Day is a holiday to reflect on past wars lest we forget the soldiers who fought for our freedoms.
烈士纪念日是一个缅怀过去战争得节日，以免我们忘记那些为我们得自由而战得战士。
At Least
至少，起码
The English expression at least can be used before/with a verb, signifying that something can be done despite circumstances, or before/with a number/quantity, signifying a minimal quantity.
在英语表达中通常用在动词前或者动词连用，表示不管情况如何都专业做某事;用在数量前，表示最小值
Although ill, at least I am here.
虽然我生病了，至少我还在这儿。
I wish to buy at least three presents.
我希望我能买至少3个礼物。
最后我们来总结一下
least是一个形容词，
意思是“最小得;最少得”，也就是little得很可以
lest则是连词，
“唯恐，以免”得意思
他们两个只是长的像而已
而“至少”则是at least
