Minecraft – 1.11 testfor mob’s health

Your problem is the space between @p and [r=3].

Try the following command:

/testfor @p[r=3]

While you cannot use testfor to do this, you CAN use execute detect. This is most likely how it would look: /execute @a ~ ~ ~ detect ~1 ~ ~ grass 0 <command>

the execute command will detect a grass block to the left (in the X cord) of all players in the world. the command is how you would track that player if it detects the block successfully.

an example tracking command would be /execute @a ~ ~ ~ detect ~1 ~ ~ grass 0 scoreboard players set @p Sample_Obj 1

comment if you have any other questions.

Source: http://minecraft.gamepedia.com/Commands#execute