Frequency:

Amount of time after which each IP SLAs operation is restarted.

Timeout:

Timeout sets the amount of time an IP SLAs operation waits for a response from its request packet.
When there is no response within the specified timewindow, the IP SLA operation will increment the Failure counter.

Threshold:

The threshold is very similar to Timeout. It must be less than or equal to Timeout value. Threshold is boundary value measured ove the operation RESULT (e.g. RTT, or jitter value collected during the opearation). Crossing threshold usually means SLA contract violation. Note that these two values apply to DIFFERENT objects.
Example:
You have configured a 10 seconds timeout and a 5 seconds threshold. When the response time is greater or equal to 5 seconds the threshold can activate an SNMP trap or another IP SLA for example. This will cause an SLA violation though the track connected to the IP SLA will stay OK, if there is a response within 10 seconds.

Configuration:

Default values:

Threshold: 5 seconds
Timeout: 5 seconds
Frequency: 60 seconds

The following rule applies to timers: Threshold <= Timeout <= Frequency
You might have difficulties when you configure Timeout/Frequency first… remember that after creating the “operation” the default values are already configured so you cannot configure smaller Timeout than 5 seconds. ┬áTry configuring in order:
Threshold –> Timeout –> Frequency