If this is the case, you should not be asking in a forum that deals with
Outlook, but should be asking in a forum that deals with Yahoo or gmail.

In general, there is no way other than forwarding each message to move mail
from one web mailbox to another.

Well, let's review.  You are asking about how to move items between folders.
Where are those folders you never mention so it could be folders shown in
the message store for Outlook or folders defined in the webmail service at
Yahoo and Gmail.  Since you are in an Outlook newsgroup, and without anymore
details than provided, yes, you were asking how to use *Outlook* with Yahoo
and Gmail.  Gmail provides POP3 access but Yahoo does not unless you pay
them for their premium service.  That means you were using Outlook - because
that is WHERE you are asking your question - and you were accessing Yahoo
and Gmail using POP3 from Outlook - because, again, you were asking your
question in an Outlook newsgroup.  That would be the conclusion of the
environment you were using since you provided no other details to come to a
different conclusion.

With Gmail, you can access it using POP3.  With Yahoo, you can access it
using POP3 if you pay them, or you can use YahooPOPs which runs as a local
proxy as a POP3-to-HTTP converter that lets your POP3 client (Outlook)
connect to Yahoo's HTTP webmail service.  However, POP3 clients *only* know
about the mailbox (aka Inbox).  They know nothing about "folders" or other
special holding areas provided in webmail services.  POP3 only lets the
client send commands to the POP3 mail server to list, retrieve, and delete
mails that are in the mailbox, not anywhere else.  RFC 1939 that defines
POP3 lists the commands that a POP3 client sends to the mail server, and
none of them involve folders and they only target the mailbox (which users,
and services, call the Inbox to mislead into thinking it is associated with
the other webmail folders).  Unless Gmail provides an import function, and
unless Yahoo provides an export function, and the output from Yahoo is
readable by Gmail, the only choice left is to forward mails in Yahoo's
"folders" area to your Gmail account where you can move them to Gmail's
"folders" area.

If I get you right this time, you want to move mails from Yahoo's
server-side webmail folders (which are not the Inbox) to Gmail's server-side
webmail folders (which are not the Inbox).  Cannot be done with a POP3
e-mail client because those folders are unreachable by the POP3 e-mail
client.  That's why I suggested you connect to them using Outlook via POP3
and yank your mails (after moving them into the server-side Inbox using the
webmail interface to Yahoo and Gmail) and put them where you want in folders
within Outlook.  After all, you are asking in an Outlook newsgroup so I
figured Outlook was involved somewhere in your dilemma.

Webmail services are unique in their configuration and what software they
use to implement their service.  They are not compatible even if they used
the same webmail software unless the services make them compatible.  As yet,
I've not heard of Yahoo or Gmail providing an export function (and one that
provides a file format that both of them understand).  Using the webmail
interface, moving the items to the Inbox folder there, and forwarding them
is your only option if you don't want to involve Outlook to manage your
e-mails locally.