I have 3 sites connected through an IPSec tunnel
Site A to Site B: 192.168.1.0/24 <--> 10.0.1.0/24
Site B to Site C: 10.0.1.0/24 <--> 172.20.10.0/24

How can I route the traffic from Site A to Site C through the IPSec tunnel?

First of all, ipsec is not a routing protocol, it is a policy.
If you can ping all the addresses in your network you're ready to implement ipsec. So you have to solve that first.
Then https://wiki.mikrotik.com/wiki/Manual:I ... n_Examples

Menu ip -> ipsec

• Configure peer

• Configure identities

• Check active peers

• Configure proposals

• Configure policies

• Check policies if there active and check with ping / traceroute with source and destination ip to activate the policy

Ipsec is already working.
But how can I setup the site A to reach the site C ?

Can you post your configs?
Don't forget export hide-sensitive

I think the case is clear without the export.
Site A connected over an IPSec to Site B and Site B connected over an IPSec to Site C.

In IP/Routes you will set Dst. address of the machine you wish to reach or the subnet and you will choose the appropriate gateway .