Not just at the same time but in 'modes.
Yes - all at the same time - just not eternally before the incarnation.
As a Father to Jesus - as His creator God Who remained Spirit as He always has been even as He also dwelled and always will dwell for the rest of eternity as a man. He was addressed as Father by the man Jesus just as we other humans see and address God.
As a Son as a human Who was created and sustained by God just as we are. A human Who was God as well but Who had emptied Himself of divine prerogatives in order to live and overcome as the man that He was and Who prayed to His Father constantly.
As a Spirit just as God has always existed and will always exist.
Where's the heresy in that?
Just because they see the one true God person beside Whom there is no other as exactly that and not as an alleged God the Son person existing beside or within a God the Father person with a God the Spirit person there with them in eternity past - doesn't make it heresy.
As for their baptism in the single name of Jesus way to salvation doctrine - I of course disagree with salvation through water baptism.
But, as they quickly and correctly point out, Jesus told His disciples to baptize in the Name of the Father, and the Son, and the Holy Spirit. Then when the apostles baptized it was in the name of Jesus - Who is at once the Father, Son, and Holy Spirit - just as He and the scriptures claim Him to be.
While the Trinity is a legitimate view of the mystery left to us considering God's nature - it is not the only legitimate view.
One can expound their own view of the scriptures we have to deal with on this subject without condemning others.
We should never condemn those to Hell who view this mystery a little differently than we happen to. Nor should they condemn Trinitarians to Hell for failing to confirm God as One person but rather teaching that He is 3 persons.
If one side denied the full divinity of the Son or the Father or the Holy Spirit - that would be another kettle of fish entirely. But neither side does.
P.S.
I've always found it interesting that Trinitarians are so willing and eager to condemn so called modalists as heretics when a full 95% of those who subscribe to the Trinity would and do give a modalistic description of the doctrine of the Trinity when pressed to do so.
On the other hand Oneness proponents willingly and eagerly condemn Trinitarians as heretics when 95% of them cannot give a proper definition of Oneness theology when they are pressed to do so.