The purpose of this study is to evaluate the efficacy, safety, tolerability and pharmacokinetics of NMD670 in the treatment of ambulatory adults with spinal muscular atrophy type 3

Inclusion Criteria:

1. Participants with a clinical diagnosis of Type 3 SMA.
2. Participants who are ambulatory, defined as being able to walk at least 50 metres without walking aids at screening during the 6-minute walk test.
3. Participant with genetic confirmation of diagnosis (e.g., homozygous deletion or compound heterozygous deletion and mutation of survival of motor neuron 1 gene [SMN1])
4. Participant with 3 to 5 copies of survival of motor neuron 2 gene [SMN2].
5. Participant has a body mass index (BMI) within the range 19-35 kg/m2 (inclusive).
6. Participant is male or female.
7. Contraceptive use by men and women must be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
8. Participant is capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol.

Exclusion Criteria:

1. Participants with prior surgery or fixed deformity (scoliosis, contractures) which would restrict ability to perform study-related tasks.
2. Participants with other significant disease that may interfere with the interpretation of study data (e.g., other neuromuscular or muscular diseases).
3. Participants with other significant clinical and/or laboratory safety findings that may interfere with the conduction or interpretation of the study
4. Participants received treatment with an investigational medical product (IMP) within 30 days (or 5 half-lives of the medication, whichever is longer) prior to Day 1.
5. Participants with history of poor compliance with relevant SMA therapy.