Replace private AS path but keep the prepending

Perhaps you're looking for the behavior of "as-override" option.
https://community.fortinet.com/t5/FortiGate/Technical-Tip-BGP-allowas-in-enable-or-as-override-when-local-AS/ta-p/197448
https://community.fortinet.com/t5/FortiGate/Technical-Tip-Replace-as-path-chain-in-BGP-advertisement/ta-p/211672

However I have more fundamental questions what you're trying to do.
1) if BGP peerings are only between your three FGTs, why do you need public ASNs? Do you have to peer with your ISPs as well?
2) The topology is hub(fg1) and spoke(fg2 and fg3) and no redundant paths. Then, why do you need to prepend?

Toshi

hi @Toshi_Esumi thank you for your reply, i will give it a try.

This is in a lab environment. Essentially AWS would not honor AS prepending if the ASN is private. so i'm going to set public ASN in one location and prepend the private ASN but i want that AWS will see the prepending as public ASN 

I'm still unable to dynamically replace the private ASN.

The idea is that my customer (FG2) will be able to prepend as many private ASN they would like and that my FG (FG1) will replace the private ASN with public ASN based on the amount of Private ASN received from FG2.

Any suggestion?

@Toshi_Esumi thank you so much!
I was able to get to work using the following settings:
FG1:

FortiGate-VM64-KVM # config router bgp

FortiGate-VM64-KVM (bgp) # show
config router bgp
set as 1111
set router-id 10.0.1.1
set graceful-restart enable
config neighbor
edit "10.0.1.2"
set advertisement-interval 1
set bfd enable
set soft-reconfiguration enable
set remote-as 3333
set keep-alive-timer 1
set holdtime-timer 3
next
edit "10.0.0.2"
set advertisement-interval 1
set allowas-in-enable enable
set bfd enable
set soft-reconfiguration enable
set remote-as 64514
set local-as 64512
set route-map-in "FROM-FG2"
set keep-alive-timer 1
set holdtime-timer 3
next
end
config redistribute "connected"
set status enable

On FG2:

config router route-map
edit "PREPEND"
config rule
edit 1
set set-aspath "1111 1111 1111"
next

config router bgp
set as 64514
set graceful-restart enable
config neighbor
edit "10.0.0.1"
set advertisement-interval 1
set bfd enable
set soft-reconfiguration enable
set remote-as 64512
set route-map-out "PREPEND"
set keep-alive-timer 1
set holdtime-timer 3
next
end
config network
edit 1
set prefix 172.16.2.0 255.255.255.0
next

on FG3 i can now see the following:

FortiGate-VM64-KVM # get router info bgp neighbors 10.0.1.1 received-routes
VRF 0 BGP table version is 1, local router ID is 172.16.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight RouteTag Path
*> 10.0.0.0/30 10.0.1.1 0 0 1111 ? <-/->
*> 10.0.1.0/30 10.0.1.1 0 0 1111 ? <-/->
*> 172.16.1.0/24 10.0.1.1 0 0 1111 ? <-/->
*> 172.16.2.0/24 10.0.1.1 0 0 1111 64512 64514 1111 1111 1111 i <-/->